Plain English Meaning
The official book from the aircraft manufacturer that tells the pilot exactly how to operate that specific airplane -- how to start it, fly it, what its limits are, how it performs, and what to do in an emergency.
Full Definition
The Pilot's Operating Handbook is the manufacturer-produced document that contains the operating procedures, limitations, performance data, systems descriptions, and emergency procedures for a specific make and model of aircraft. For aircraft certified after 1975, it is generally formatted to the GAMA (General Aviation Manufacturers Association) standard and, when an FAA-approved Airplane Flight Manual is required, the POH and AFM are typically combined into a single document.