Energy is a vast topic which is highly relevant to all living beings. We use energy to be able to do everything that we do, from hard physical exercise, to digestion, and even to mental energy that we use for things like reading, studying, planning things and even used and exchanged in conversations or interactions with others, so if I had to describe energy in one word, I would call it "Attention", and you may have already come across a saying, to the likes of "Where attention goes, energy flows", because we connect to things with our consciousness. Quantum physics today tells us that the simple act of observing something creates a connection to that object.
Being such a valuable and necessary resource, we must protect our energy much like one must protect their food and their finances from those who seek to take from others. Many human beings are low on energy and have never learned any techniques to increase their energy levels so they have naturally and unconsciously learned how to take the energy of other people who are more lively and vibrant. If you feel depleted at the end of a conversation with someone, this may be a red flag indicating that the conversation was not a constructive or positive one, and that, instead, the energy spent on the conversation was sent away to something that will not feed it back to us.
Think of energy like an investment, If you put all of it into something that is irrelevant to you or your development, such as complaining with other people about something that you plan on doing nothing to change, or gossiping about the activities of other people, or being judgmental about things we know little about, your energy will be wasted and move away from you and into something else, generally that thing that you were talking about. this is why it doesn't serve us to complain about things, it is much better to create a plan to make a difference, invest that time and energy into yourself if you want to see yourself grow.