It's one of the most famous misquoted lines in the English language, spoken by Lady Gertrude (Hamlet's mom) during the intense scene featuring a play ("The Mousetrap" aka The Murder of Gonzago) within a play, intended by Hamlet to be "the thing wherein I’ll catch the conscience of the King." The character in "The Mousetrap" has delivered some sappy lines about commitment and widowhood, designed to flush out the King and Gertrude.