An APO/FPO address is one that's used to send mail to members of the U.S. Army, the U.S. Air Force or the U.S. Navy. APO stands for Army/Air Post Office, and FPO stands for Fleet Post Office.
The APO and FPO addresses are part of an agreement between the U.S. Postal Service and the Department of Defense called Overseas Military Mail, which was established to get mail to and from military personnel overseas. Along with the post office designations, three military "states" were created: AA, which stands for Armed Forces Americas; AE, which stands for Armed Forces Europe; and AP, which stands for Armed Forces Pacific. The processing center for AA mail is in Miami, Florida, and the center for AE mail is New York, New York. AP mail is processed in San Francisco, California.
Military shipments must have the full names, grade and unit number of the recipient. Shipments meant for the Navy must include the ship name or fleet number. The USPS assigns special ZIP codes to military bases and organizes them by country or region.