The meaning of this phrase is to alert co-workers, on-lookers, passers-by, and anyone else in a group, large or small, who is in charge of a given scenario.
Bryan: Well, what if we approach this problem from a different perspective?
Debra: I'm fucking this cat, so just shut up and hold its tail!
A better way to ask, "fish or cut bait" or "piss or get off the pot"; in other words, to tell someone to be decisive in making a decision.
Literally it probably refers to a situation where you're competing for a woman with another guy.
In other words: "Are you going to fuck her(ie. win), fight him(ie. TRY to win), or hold a light up for his convenience while he fucks her?(ie. be a total loser)"
Sort of a takeoff on the whole "fight or flee" natural evolutionary response.
John: Yo, Ron, it's your turn to bet!
Ron: I'm thinking
Ray: Enough thinking, already! Look - either fuck, fight, or hold the light!
In more tribal times, one tribe would attack another just before dawn, when it is darkest (thus the true mean of its always darkest before the dawn). Rape was one of the traditional spoils of war, so members of the invading force would either rape the women of the opposing tribe, fight the opposing tribes soldiers, or hold the light so other soldiers could see who they were attacking and raping.
HeyIvan, don't just stand there like an idiot, these Nazis bitches aren't going to rape themselves: fuck, fight, or hold the light.