Plain English Meaning
The brakes on the main wheels that the pilot uses to slow down on the runway, control speed while taxiing, and hold the airplane still during engine checks.
Full Definition
The wheel-mounted friction devices on an aircraft's main landing gear that slow or stop wheel rotation, allowing the pilot to decelerate during landing rollout, control taxi speed, hold the aircraft stationary during runup, and assist with directional control on the ground. Most light aircraft use independent hydraulic disc brakes operated by toe on the top of each , with each controlling its respective main wheel.