MGU-K stands for Motor Generator Unit Kinetic. It is part of the hybrid system. The unit recovers energy during braking. It also provides electrical power to the engine.
For example, when braking, the MGU-K charges the battery. Later, it delivers extra horsepower on acceleration. It connects directly to the drivetrain.
The MGU-K improves both efficiency and speed. Managing it correctly boosts lap performance. It is a vital hybrid component.