Cells use chemical energy to do just about everything- to power chemical reactions, to build new molecules in order to survive, and to perform life processes.
Cells have a very specific energy source known as adenosine triphosphate, or ATP. When ATP is transformed into a compound called ADP, it releases energy that will be used by the cell. This is because chemical energy is stored in bonds, and a high energy bond is found in ATP. When that bond is broken, energy is released. A phosphate group is lost in the process, which transforms adenosine triphosphate into adenosine diphosphate.
In order for the outer phosphate bond to break, an enzyme called ATPase helps speed things up via hydrolysis. Hydrolysis is a mechanism in which a bond is broken by adding water (hydro=water; lysis=to break). Just as water was removed to create the bonds, it is added to break the bond.
This transformation is not permanent though. There is a cycle among ATP and ADP. The energy required to complete this cycle is supplied by cellular respiration, specifically energy from glucose molecules that you obtain from food.