I looked it up. :-) Literally, "once only". In English the more usual expression is "only once" or "only one time". However, when "kerran vain" begins a passage (as it often does), then to me it has the sense of "once upon a time".
Nothing to do with "once upon a time" though. In finnish the order of the words is very free. Basic order for these is "vain kerran" and the meaning is the same.