Often used to refer to a grouping of starting hands, usually in the context of speaking of an opponent's likely holdings. For example, if a tight player decides to reraise a preflop bet, that action likely narrows his "range" of possible hands. If he keeps betting on later streets, that further narrows his possible hands depending on how the community cards come.
When discussing strategy, it is often more feasible to talk about an opponent having a "range" of possible hands than a particular hand. That is to say, it usually makes more sense to conclude from an opponent's betting that he has, say, "a range that includes big pairs and high cards" than to pin down precisely one particular hand he might have.