Final answer:
The green EXT POWER AVAIL light indicates external power is connected to the airplane.
Step-by-step explanation:
The green EXT POWER AVAIL light on the overhead electrical panel indicates that external power is connected to the airplane. This means that the airplane is receiving power from an external source rather than relying on its internal power supply. It is commonly used when the airplane is on the ground and needs to power its systems, such as during maintenance or while passengers board or disembark.