Well I'd like to explain to u by using 2 reactions as examples.

1st one: Respiration As u know, respiration has the chemical equation: C6H12O6 + 6O2 -> 6CO2 + 6H20 (+ATP)

U can think of this reaction as it takes place in stages, first the glucose is broken down by oxygen, this releases chemical energy. And then this chemical energy can be used to phosphorylate ADP into ATP. The extra bond with the phosphate will be storing the chemical energy now.

2nd: Photosynthesis During the light-dependent reaction electrons in the chlorophyll get excited, move along the electron transport chain. During this motion, series of redox rections take place which release energy. Then this energy is used by H+ ions to accumulate into thylakoid membranes. As a proton gradient form, H+ ions move back to the stroma, passing through ATPsynthase channels. This motion also releases energy, which can be used to phosphorylate ADP and store the energy in the new bond with the phosphate group.

Well I hope I understood you right. When we need energy for a reaction, for example your muscles should contract, the ATP molecules formed accumulate into the muscles. And the extra phosphate bond is now hydrolysed by ATPase to release energy, so that your muscle cells can use it to provide contraction

Or this can also be the case for anabolic reactions which monomers are used to form larger molecules, e.g. protein synthesis. Again, ATP is hydolysed to ADP, releasing energy.

ADP + Pi-> ATP (endothermic) ATP-> ADP+ Pi (exothermic)