Heat is a form of energy.
If you are warm blooded, some of that energy is converted to heat to keep your body the same temperature all the time. If an animal is cold blooded, they take on the temperature of their surroundings so they don't have to use food energy to keep warm. This means they don't have to eat as often. But it also means that when it is cold, their bodily functions slow down. There is good and bad to both methods.