Plain English Meaning
On a taildragger, it's the back wheel that turns when you push the rudder pedals, letting you steer the airplane while taxiing.
Full Definition
A small wheel at the rear of a tailwheel-equipped airplane that is mechanically linked to the rudder pedals, allowing the pilot to direct the aircraft on the ground by pressing the pedals. The linkage typically engages within a limited arc of pedal travel; beyond that arc, the wheel either disengages into a free-castering mode or remains locked depending on the design.
Context Anchor
Seen when taxiing, taking off, or landing a tailwheel airplane, especially while learning how the airplane responds on the ground.