This line is the Queen's response to the speech given by an actor portraying a queen in The Mousetrap. The queen in the play has just learned that her husband, the King, is dying, and she promises that she will never marry another, because to do so would be to betray her first love. Obviously, this would make the Queen unhappy, and so she says the player queen is protesting too much. Hamlet's response is pithy and acidic: "O, but she'll keep her word." The speech, and her son's comment, must have struck a nerve with the Queen, and Claudius responds by asking Hamlet if the play will be offensive.