These are motors
with a built-in brake disc and an electromagnetic mechanism. The brake disc is just like the automotive counterpart: a metal disc rigidly mounted on the shaft which is pressing the braking pads. It brakes by transforming the kinetic energy of the rotating motor+load into heat (another form of energy) using the friction between the pads and the disc. The heat is then dissipated away naturally and the target is reached: the motor is stopped. The brake is used for rapid deceleration or for immobilizing the motor after removing power. The brake is controlled either directly with DC or via a rectifier mounted usually in the conduit
box. The simplest implementations have the electromagnetic brake fed from the same wires as the motor, so there is electricity holding the brake in the OFF position. As soon as power is turned off, the motor stops spinning and the brake is automatically applied, immobilizing the motor. Applications for such a setup may include various machine tools or elevators (which must stay put when off).