Plain English Meaning
Someone or something is on the runway when they shouldn't be, putting an arriving or departing aircraft at risk.
Full Definition
Any unauthorized presence of an aircraft, vehicle, person, or object on the protected surface of a runway used for the takeoff and landing of aircraft. A runway incursion creates a collision hazard or compromises the required separation between an aircraft taking off, landing, or intending to do so, and any other aircraft, vehicle, or person on that runway.
Context Anchor
You will encounter this term in ground operations, especially while taxiing, crossing runways, reading runway signs and markings, or following tower instructions.