ATP is made up of 3 negatively charged phosphate groups next to each other. (As well as a nucleotide and nitrogenous base too). These phosphate groups are bonded together but greatly repulse each other. The bonds can be broken by hydrolysis and this releases a lot of energy due to this high repulsion. In this reaction, ADP is formed. So a lot of energy is given out making ATP a "high energy" molecule.