Hi Rob, any speaker or speaker driver has a nominal pass band or frequency range within which it plays. Above and below this pass band the response "rolls off " I.e. output reduces more or less gradually.
With a crossover it's similar, the crossover point is where the output from each driver is 3 or 6 db down. Below or above that frequency the output continues to reduce or "roll off." The rate at which it reduces it usually called the slope and is expressed in db/octave.
The slope or rate of roll off in either case is important for various reasons. Suffice to say that when someone refers to a crossover frequency or frequency limit that is far from the whole story. Hope that helps, if not please provide some context for your question.
