Plain English Meaning
A skid is when a tire stops rolling and starts sliding across the ground. The wheel is no longer turning freely, so the airplane slides on the rubber instead of rolling on it.
Full Definition
On landing or during taxi, a skid occurs when a tire loses traction with the runway or taxiway surface and slides rather than rolls. It commonly results from excessive braking that locks the wheel, or from braking on a wet, icy, or contaminated surface where available friction is reduced.
Context Anchor
Seen in landing gear, tire, and brake discussions, especially during touchdown, the landing roll, braking, and taxiing.