To preface, a ghost light is one left activated on a stage even when the house is empty and no performaces or rehearsals are being conducted. It's a bare bulb that burns for no one.
Some say this is so no one hurts themself in a darkened theater, by tripping off the stage into the orchestra pit for instance.
However, theater people being what they are (to wit, a highly superstitious lot) many believe the contraption and its name come from the long tradition of burning a light for the resident ghost--every theater has one. And they're often considered good luck. Some traditions, though, hold that the light is burned to keep them at bay. I suppose it depends on how harsh a critic your resident ghost is.
