Final answer:
The green EXT POWER AVAIL light indicates that external power is connected to the airplane and is ready for use, with acceptable voltage and frequency levels.
Step-by-step explanation:
When the green EXT POWER AVAIL light on the overhead electrical panel is illuminated, it indicates that external power is connected to the airplane and not on line and operating within limits. This means that the external power source is ready to be used by the aircraft's electrical systems, and that voltage and frequency are within the specified range for safe operation. The external power source is neither inoperative, overloaded, nor disconnected at this point.