The underlying unity between the microcosm (pinDANDa: the human egg) and macrocosm (brahmANDa: the Cosmic egg) is an important concept for sAdhaka-s. With this awareness it is easier to direct one's meditation. In the article on Knowledge and Gnosis it was noted that the mind and sense organs are not reliable ways of knowing the Eternal Truth. But we also need to have some idea of what this Eternal Truth is through the conscious mind before we can direct our efforts to develop jnAna.
This is the provisional framework from the Agama-s that was referred to in that same article. The intent is that the sAdhaka will provisionally accept this concept, subject to verification through inference (anumAna) and eventually direct perception (pratyaksha). Below is a simplified version of the microcosm and macrocosm. There are several conceptions of this, and the terminology can vary even from text to text, so please bear this in mind. I have deliberately simplified it to serve as a starting point.
The Upanishads declare that Atman (the indwelling Consciousness) in the human being identical to Brahman. 'Tat tvam asi", or 'That Thou Art' is one of the mahAvAkyas (great sayings) of the Upanishads. More details on mahAvAkyas can be found on SwamiJ's website.
Although it appears beguilingly simple, it is worth keeping in mind that many lifetimes can be spent in just gaining jnAna of what 'tvam' or 'Thou' is. Of those few who are fortunate to even know the true meaning of 'tvam' is then the charge to know 'Tat'. In fact, there is a subtle point that when the One or Truth is expressed in this form, that itself becomes a 'relative' truth and one can get into all kinds of logical difficulties! The approximation of the Truth that follows reduction of the non-dual for apprehension in the dual world is something that is inevitable in transmission through words (written or spoken).
Nevertheless, at the mental level we can understand the basic concept that essentially the Cosmic consciousness (Brahman) that manifests (takes on vyaktitva) as jagat (the world) of nAma-rUpa (name and form) is the same as the individuated consciousness (Atman as we experience it in the jIva). The key point that is to be noted is that the manifestation of Atman as the human being is parallel to the manifestation of Brahman as the world, or jagat. So the body is a miniature universe. This is a fundamental concept that needs to be understood in the practices of Yoga, VedAnta and Tantra. This reflection of the macrocosm in the microcosm is a recurrent theme that appears in the articles on SandhyA, Sacrifice and Death.
The manifested external reality that we perceive is conceptualized as consisting of five principal elements: the pancha mahA bhUtas. These are in increasing order of subtlety: pRthvI (earth), apah (water), agni (fire), vAyu (air) and AkAsha (space). At this stage it can be useful to remember that while to our minds this may seem overly simplistic, it was probably completely adequate in describing the entire external world at a time when there were no man-made materials. More to the point is the fact that it is still completely adequate for the sAdhaka on the inward journey. Remember that we are not trying to build a theory of matter in the modern scientific sense, but we are looking at a holistic theory (a theory of everything if you like Lee Bladon's characterization) that encompasses the external material world, man's internal states, and consciousness. I think it is not necessary to force holistic theories to apply in predictive detail to every aspect of external reality, and this is discussed in another article. If the goal is to seek the Truth that is beyond the mind, it is enough to have a reasonable framework that describes external reality to the extent that is needed. Call it an engineer's approach to spirituality!
We are not immediately aware of our individuated consciousness because it is enclosed in five sheaths: the pancha koshAs. Again in increasing order of subtlety, these are: annamayakosha (the sheath of food), prAnamayakosha (the sheath of energy), manomayakosha (the sheath of the lower mind including manas, chitta and ahamkAra), vijnAnamayakosha (the sheath of the discriminating part of the mind called buddhi), and Anandamayakosha (the sheath of bliss). The goal of sAdhanA is to first penetrate these sheaths through yoga meditation and gain awareness of our individuated consciousness. Then begins the true study of the nature of the Self through VedAnta. Tantra helps along the way inward by providing us knowledge of a hidden reservoir of spiritual energy called kundalini that helps greatly in propelling us along in the spiritual journey.