A "set", sometimes called a "book", is a group of 3 or 4 cards, all with the same rank but with different suits. If the game is using more than one deck then a set usually can not have more than one card with the same suit. For example, these are valid sets:
But these are not:
9-9-9-9 9-9-6-9 9-9
A single card can only be used once as part of one set or run, and cannot be reused in different melds at the same time.