ATP is the energy currency of living organisms. Not to get too technical, but it's a molecule with three phosphate groups (ATP stands for adenosine tri-phosphate). When energy is needed, one of those phosphate groups is broken off (making ADP - adenosine di-phosphate) and the energy stored in that bond is released.
To get ATP, cells use mitochondria (the powerhouse of the cell ;) ) to convert sugars into ATP through cellular respiration. Starch is one of these sugars. Therefore, starch is used to store energy long-term, and when energy is needed it's converted to ATP. Then, processes that require energy use that ATP.
Hope that helps! Feel free to message me or leave a comment if you need clarification or anything :)
