Plain English Meaning
The pilot who is in charge of the flight. They make the final decisions about how the flight is flown and are legally responsible for everything that happens during it.
Full Definition
The pilot who has final authority and responsibility for the operation and safety of a flight. The PIC has been designated as such before or during the flight, holds the appropriate category, class, and type rating (if required) for the aircraft, and is the person legally accountable for the conduct of the flight under 14 CFR 91.3.
Context Anchor
You will see PIC used in flight training, logbook discussions, preflight planning, crew briefings, and FAA rules about who is responsible for a flight.