Final answer:
The ON light on the EXT PWR pushbutton indicates that external power is connected and supplying electrical power to the system.
Step-by-step explanation:
The question refers to the understanding of an electrical panel specifically in aviation or any other field where such panels are used to monitor the status of external power sources. The ON light on the EXT PWR pushbutton indicates that external power is connected and supplying electrical power to the system. This is a standard indication used in various types of machinery and equipment that rely on external power sources for operation. In this context, 'external power' could be provided by a ground power unit, airport infrastructure, or similar external supply. When the light is illuminated, it informs the operators that external power is successfully being routed into the system's electrical grid.