The Motor Generator Unit - Kinetic (usually abbreviated as MGU-K) is a component of a hybrid-electric power unit. The unit is both an electric generator and motor. Connected directly to the crankshaft of the Internal Combustion Engine (ICE) by gears, it performs two functions: under acceleration it helps power the engine, and under braking it uses the energy that would have been dissipated as heat to recharge the batteries. It goes through that cycle several times each lap: drawing electricity to help the engine deliver power, and generating electricity while helping the engine slow the car.
Over time, a side benefit has become apparent: modern F1 cars do not carry starter motors, but a car that has traveled in a practice session or race will have enough stored energy to re-start the engine, therefore duplicating the function of a starter.