ADP(adenosine diphosphate) is a low energy molecule and ATP (adenosine triphosphate) is a high energy molecule in the cell. In order to convert ADP into ATP, an extra phosphate molecule is required that forms an extra bond (high energy phosphoanhyxride bond) with ADP and produces ATP.
Thus, this process requires energy (endothermic) and a phosphate molecule along with ADP to form ATP.