A gap is the space between two things. It can be physical (like "the gap between your two front teeth") or philosophical (like "the generation gap" between you and your parents).
Gap is a small word that packs a lot; it can be used in a straightforward manner: in England, the signs in the subway station say, "Mind the gap" — in hopes that passengers will not wander into the space between the train and the platform. The American retailer, "The Gap," is said to have taken its name from the generation gap. The clothes were designed for teenagers and young adults, and therefore nothing your middle-aged parents would be caught dead in. Times change.