A Grosvenor ("GROHV-ner") Gambit is an intentional misplay of a hand aimed to psychologically upset one's opponents. It can be employed by declarer and defenders alike, but this description focuses on the defense.
South declares 6 and West leads his two top clubs. After ruffing the second, South cashes his two top spades and ruffs a third spade with dummy's J.
East could overruff with the Q to immediately set the contract, but discards instead. Now South incorrectly assumes that West has the queen, and cashes the top trumps. East scores the Q after all, and presumably upsets South with his antics.