Representation is describing some aspect of reality with a useful symbol (Marr, 1982). More precisely, representation includes the set of symbols along with rules that determine how the representations are applied to a particular entity.
Classical theories of cognitive science treat the brain as an information-processing system; that is, intelligent systems need a rich internal world of symbols to represent external objects and environments. This “computationalist” theory assumes that information-processing systems are representation-hungry.
It has also been theorized since antiquity—see Aristotle’s signet ring—that Reality is mediated by our mental models. Whether these models or representations have some relationship with § Intelligence is an ongoing debate.