Any word ending in-ase is an enzyme. The first part of the word tells you what is the enzyme is changing or acting upon.
So ADP needs an inorganic phosphate (Pi) to become ATP. ADP+Pi yields ATP using the synthase to make the reaction work.
ATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate (ATP). ATP is the most commonly used "energy currency" of cells from most organisms. It is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi), and needs energy.