Dharma, which derives from the root verb (धृ dhR, dharati: supports), means that which sustains and supports. One may also say that dharma is the AdhAram, or support of all Existence. Thus sanAtana dharma is that which is the Eternal Support of all Existence. Understanding the Vedic concept of dharma is not easy, which is why understanding the MahAbhArata and Bhagavad GIta are truly lifelong endeavors.
Let us look at a few simple examples to clarify what dharma means. Consider a tree, which is often praised in Sanskrit literature as the embodiment of selfless giving: it gives shade to travelers, and bears fruit that it gives up uncomplainingly. So the dharma of a tree is easy to understand. It is to grow, bear flowers and fruit, and eventually die. Next let us examine the dharma of a lion. The dharma of a lion is to kill its prey when it is hungry, to procreate, and live out its appointed length of life. Plants and animals have it easy insofar as dharma is concerned because they follow the natural law uncomplainingly. One doesn't encounter a tree that didn't want to give up its fruit, or a lion that wanted to spare its prey. So the dharma of plants and animals is easy to understand and follow. The dharma of a human being is much harder to understand because a human being has the choice of acting according to his or her free will. All confusion regarding dharma arises in a human being when either attachment or ego (more precisely, the ahamkAra: I-maker) dominate and send false messages to buddhi, which is the decision-making faculty of the mind. Caught in the dilemma of not knowing what one's dharma is, this choice of free will can sometimes almost feel like a curse. But through sAdhanA it is finally understood that this choice is the great blessing bestowed on humans. For although it is easy for plants and animals to follow their dharma, they also do not have the capacity to choose their destiny and realize the Truth. This is why the human birth is extolled as a great blessing and achievement that should not be squandered.
There is a primary dharma of every human being endowed with consciousness, and I will call this svadharma (sva means one's own, so svadharma is one's own dharma). This primary dharma, or svadharma, is the right to practice sAdhanA so that the jIva (individual soul) is led to union with the paramAtman. In other words, to know one's true nature or to experience the Truth is everyone's svadharma. Sometimes the word svadharma is used in a slightly different context in the Bhagavad GIta, so please note that I am using it in this limited sense for the purpose of explaining dharma in an easier way. The reason this svadharma is of primary importance is that it is this dharma that yokes into place all the other dharmAs that sustain Creation. No other commitments to family or work take precedence over this because one is a human being first, and then come all the other roles that one has to play. These secondary dharmAs all fall properly into place when the primary svadharma is honored and fulfilled. Foolish people first attach to the other secondary dharmas while neglecting svadharma, and are confused as to what dharma is and hence, how to act.
Dharma does not derive from societal norms, nor does it derive from religion. A yogi who is established in svadharma may act in apparent contradiction to societal and religious norms. A prime example is Sri Adi ShankarAcArya. He left his widowed mother at an early age to pursue his svadharma, but in doing so he was not violating his dharma towards his family. Had he done the same thing for a selfish reason then that would have been a different matter altogether. That is why it is said that the actions of yogis are neither white nor black:
karma ashukla akRshNam yoginah trividham itareshAm (Yoga Sutra 4.7)
The actions of yogis are neither white nor black, while that of the others is threefold.
The concept of dharma has been distorted in many ways by some people so that they can manipulate the actions of others through requiring social or religious conformity. It is extremely important to know when other people are expecting you to do something for them out of their own attachments, and how they distort this expectation and call it a dhArmic obligation. Perhaps in the Satya Yuga when everyone was following their svadharma there was sense in equating dharma to societal norms. In Kali Yuga we often see that sometimes even a close family member may not know their svadharma, and may even thwart a sAdhaka's pursuit of svadharma. A direct consequence of this is that a sAdhaka should honor the svadharma first. If possible the sAdhaka should gently negotiate the relationship that consistently poses obstacles to progress on the path. By this I do not mean that one should allow one's ego to take charge and order people around on the pretext of fulfilling one's sAdhanA. But if any relationship with another independent adult---be it familial, spousal, or filial---poses consistent obstacles to spiritual progress, then the sAdhaka must have the strength to at least temporarily rearrange (or if necessary even sacrifice) that relationship in order to preserve his or her svadharma. As sAdhana progresses the aspirant will eventually be able to accept anything that life brings, but some skill is needed in the early stages when this spiritual strength is being built. Just as one does not try out for the Olympics in the early stages of learning a sport, similarly the aspirant needs to know when certain challenges can be faced, and when they should be deftly sidestepped.
Once one is firmly rooted in svadharma, then one is naturally bound by the yamas and niyamas. Therefore, one cannot cause harm in any way or form (physical, verbal or mental) to another human being since one is bound by ahimsa (non-harming). Through one's sAdhanA arises a deep compassion for the avidyA (ignorance) that all human beings are subject to. This naturally leads to forgiveness as one realizes that it is only the play of the three guNas, or prakRti who is the Actor, and not any other human being. A sAdhaka also seeks to reduce the karma that binds by performing selfless action. So long as one is loving and practicing yamas and niyamas, such a yogi makes all decisions in accord with the Divine will. Dharma to such a yogi is self-evident. For such a person there will be no confusion regarding dharma, and no conflict between pursuing svadharma and other obligations to family, work and community. The secondary dharmas align themselves naturally once svadharma is established.
All objects of creation must fulfil their dharma, and this is what sanAtana dharma means. Just as the sun, moon, plants and animals fulfill their dharma, so must man. When man acts according to his dharma, he is directly informed by Rta or the Truth. This is why in the Bhagavad GIta in the chapter on Kshetra and Kshetrajna (the Field or the Body, and that which knows the Field, or Consciousness) [XIII.9-11], it is said
Humility, unostentatiousness, nonviolence, forbearance, uprightness, service to the Guru, purity, steadiness and self-control; non-attachment to the objects of sense gratification, absence of egoism, seeing misery in birth, death, old age and disease; nonattachment to children, wife, home and the rest, and even-mindedness amid pleasant and unpleasant events; constant and unalloyed devotion to Me, resorting to solitary places, detachment from the general mass of people; accepting the importance of self-realization, and philosophical search for the Absolute Truth--all this is wisdom, and what is different from these is ignorance."
Since these qualities lead man directly to act according to his dharma, they are considered to be jnAna or wisdom. Knowing that he is also a manifestation of the Divinity, man can learn to act properly in the world. This wisdom comes through sAdhanA.