Adenosine diphosphate (ADP) is a molecule that is a source of energy. But it is less than what ATP can give. ATP is adenosine triphosphate. It is being produced when the cell, primarily at the mitochondria, adds a phosphate group to ADP.
ADP + P = ATP -> high source of energy
ATP now becomes a strogare of energy which will be used for the metabolism of the cell. We can also term this as ATP-ADP cycle. When ATP is broken down to ADP, energy will be produced. The breaking down of the phosphate group is used to build and move things inside the cell in the form of chemical reactions.
ATP -> ADP + P + energy (the reverse of the former cemical equation)
hope this helps :)