Plain English Meaning
Small wedges put against the wheels so the airplane cannot roll forward or backward when it is parked or when the engine is being started by hand.
Full Definition
Wedge-shaped blocks placed snugly against the front and/or back of an airplane's tires to prevent the airplane from rolling while parked or during ground operations such as hand propping.
Context Anchor
Seen during parking, preflight, engine starting, and especially hand propping procedures.
Derivation
Chock' is an old English/nautical word for a wedge or block used to hold something steady, originally used to keep barrels and small boats from rolling. The aviation use carries the same idea over to airplane wheels.