Modern cars and trucks use brake rotors (also called brake discs) on the front of the vehicle. They use brake rotors or brake drums on the rear to slow down or stop the vehicle when the brakes are applied.
The brake pads and rotors operate in a similar way to how you would use your hands to slow or stop a wheelchair. In the same way you grab the wheels of a wheelchair, the brake pads grab the brake rotor, thus slowing down the vehicle.
When the brakes are applied to slow down the vehicle, heat is produced. When there is excessive heat, the brake rotors can warp. If the brake rotors are warped even three or four one thousandths of an inch, the brake pedal may pulse and the vehicle may shake when the brakes are applied. If the brake rotors are badly warped, the whole vehicle may shake and cause the steering wheel to oscillate and shake from side to side under braking.
To eliminate this unwanted vibration, the brake rotors need to be measured both for uniform thickness (parallelism) and "run-out." Run-out is measured by rotating the brake rotor and accurately quantifying how much deflection the surface of the brake rotor takes as it is rotated. The brake rotors are brought back to uniform thickness and parallelism by "machining" (if there is sufficient material left to remove). If the brake pads have worn down to the point where they have damaged the brake rotors, then the rotors will need to be machined or replaced.
Warped brake rotors or brake drums are removed from the vehicle and machined on a brake lathe. Brake rotors or drums are only machined if they are sufficiently thick enough to have metal removed and still be serviceable. Brake rotors on front wheel and all wheel drive vehicles may be machined on the vehicle using a special brake lathe.
Vibration felt under braking may also be caused by worn suspension components, worn bushings, or bad shock absorbers.