Plain English Meaning
It's the part of the landing that happens after the wheels touch down — the rollout down the runway while you're slowing the airplane and keeping it tracking straight.
Full Definition
The phase of a landing that begins the moment the airplane's wheels are firmly on the runway and ends when the airplane is slowed to taxi speed or comes to a stop. During this phase, the pilot continues to actively fly the airplane on the ground, using rudder for directional control, brakes as needed, and proper aileron and elevator positioning to counter wind effects.
Context Anchor
Encountered during landing practice, especially after touchdown when the pilot is using steering, brakes, and to aligned with the runway.