ATP is used like a carrier for energy in the body. It's Tri(3)ose phosphate, when one phosphate breaks off it becomes Di(2)phosphate, when the bond breaks it releases a little bit of energy. It's needed because it can release little bursts of energy not massive amounts all at once like a starch molecules would do. It's used for active transport, muscle contraction etc. in the body and is produced by respiration (the reaction you mentioned). You'll learn much more detail about it at A-Level if you take it, for GCSE you really just need to know that it carries energy for reactions in the body