Final answer:
The Ground Power Unit (GPU) provides electrical power to an aircraft on the ground, enabling operation of its systems without using the plane's engines and preserving the aircraft's batteries.
Step-by-step explanation:
When the Ground Power Unit (GPU) comes online, it provides electrical power to an aircraft while it is parked on the ground. The GPU allows the aircraft systems to operate without the need for the aircraft's engines to be running. This external power source is particularly useful during maintenance, when loading or unloading passengers and cargo, or to power up the aircraft systems before engine start. When connected, the GPU supplies the necessary voltage and frequency to the aircraft's electrical buses through a cable, ensuring that the aircraft's batteries are not drained and electrical systems are fully operational for checks and setups.