Final answer:
When the GPU AVAIL light is on, it means the bus is connected to an external power source. The buses on the aircraft can be connected to the ground power source to receive electrical power.
Step-by-step explanation:
When the GPU (Ground Power Unit) AVAIL (Available) light is on, it means that the bus is connected to an external power source. The GPU provides electrical power to the aircraft when it is on the ground, allowing the aircraft to operate without using its own engines or APU (Auxiliary Power Unit). The buses on the aircraft, such as the AC (Alternating Current) and DC (Direct Current) buses, can be connected to the ground power source to receive electrical power.