This powers your built-in shield system on your ship. When you are out of energy, one hit will probably kill you. If you do have energy, the energy is depleted by the hit, but your ship/character is not destroyed or killed.
It is usually the case that when you add energy to a bunch of atoms they move faster and get hotter. Similarly, if you remove energy from a bunch of atoms, they usually move less and get cooler. Because adding heat energy usually results in a temperature rise, people often confuse heat and temperature. In common speech, the two terms mean the same: "I will heat it" means you will add heat; "I will warm it up" means you will increase the temperature. No one usually bothers to distinguish between these. Adding heat, however, does not always increase the temperature. For instance, when water is boiling, adding heat does not increase its temperature. This happens at the boiling temperature of every substance that can vaporize. At the boiling temperature, adding heat energy converts the liquid into a gas WITHOUT RAISING THE TEMPERATURE.