ATP is short for adenosine triphosphate, which means that there is an adenosine molecule (A) (don't worry too much about that for the moment) with three phosphates (P) joined onto it. ATP could be written at APPP. ADP stands for adenosine diphosphate, which means the A has 2 phosphates joined to it, and might look like APP.
Therefore,
APPP -> APP + P
ATP can become ADP plus a free P (because one broke off)