Energy is, roughly, a force that exists everywhere which keeps the gears of the universe turning. Energy is a substance which will act based on the will which is placed into it. Energy is like a car- the key needs to be inserted and turned, the gears set, the gas pedal pressed, and the wheel turned. With no will or intent behind it, energy does nothing. A car without a driver will never fulfill its intended purpose.
Now, of course humans are not always or even usually the primary facilitators of this purpose. The will that molds energy is often provided by the spirits that exist everywhere, throughout the entire cosmos. In our lives though, as spiritual seekers, we can learn to harness our minds and souls in order to mold the energy, if to a lesser extent when compared to the great spirits themselves.
Our own spirits and souls are made up of this energy, we were initially formed to be what we are by powerful spiritual entities. Thanks to this, you are self aware and have the ability to choose your fate. The energy merely floating around however, is not self aware and needs guidance to do anything. You can achieve this by focusing your will into the energy. This will cause it to move towards a goal of your choosing.
Energy is limitless as far as what it can do goes. You, however, are not limitless while existing within humanity. The physical body imposes limitations on your being which are meant to cause you to grow as a spirit. So while the energy itself has no limit - what you as a human can ultimately do with it is limited.
You can learn to mold the energy to your own will by focusing your will and the intent behind it. This is often achieved through meditation, but ritual spell casting or ceremonies are also used to attempt to mold the energy to the will. Whatever it is you are doing, you can be assured that success is found when you mold the right energy.
Know that your thoughts, feelings, desires etc are all put out into the energy if you focus on them strongly enough. Therefore, getting depressed and fixating on how terrible you feel will cause the energy around you to be negative for you. Being thankful and focusing on what you do have in the present will cause the energy to be positive. When casting spells or meditating on the energy it is important to remember that keeping a disciplined mind will be of great benefit to you when you want to work with the spirit realm.