In theatre, what is a ghost light?

Many theatres will leave a ghost light on stage mainly for safety. Popular myth likes to highlight the superstition of actors in wanting to either keep away the ghosts that are surely there or possibly comfort them with light. In reality, falling off a stage in pitch dark could indeed lead to the "broken leg" they often wish to each other!

https://playbill.com/article/ask-playbillcom-the-ghost-light-com-153440

"And in an empty theater, with the ghost light on and the darkness, warm and velvet like a dinner jacket my father once wore, it's mine."
~Christina Haag.