dont fire til you see the whites of their eyes

Dont fire til you see the whites of their eyes is an expression which is advising someone not to act before they have a chance of success. Literally speaking, this cliche is talking about not firing (shooting a gun) at their opponent until they are so close they can see their eyes because this means they are close enough tot heir target to be successful. Figuratively, it can mean any type of act which you need a good amount of success for.