Plain English Meaning
The strong internal beams that form the backbone of the wing. They are what actually hold the wing together and carry its load in flight.
Full Definition
The main lengthwise structural members inside an airplane wing that run from the wing root toward the wingtip and carry the primary bending and lifting loads of the wing. Ribs, skin, fuel tanks, and control surfaces are all attached to or supported by the spars.
Context Anchor
Seen in weight and balance, aircraft loading, and structural-limit discussions, especially when explaining how and flight loads are carried by the airplane.