ATP is an unstable energy molecule. The Three phosphates in the tri phosphate molecules are pushed together like a spring. This requires energy which is release when the third phosphate is released.
The cell uses chemical energy released by the formation of water to push a third phosphate atom on the ADP molecules ( D, di means two). This creates the unstable energy molecule ATP.
The ATP molecule is then like a charge card with cash stored in it.
This stored energy can be released for use by the cell when the third phosphate molecule is released. This reforms ADP. The ADP is like an empty charge card with no money on it. But the ADP can be recharged again by the expediture of energy