'tu quoque' is a logical fallacy. It is performed by responding to the accusation "You did X" by saying "Maybe, but you did X, too." Whether or not the accuser did X is irrelevant to the truth or falsehood of the original accusation. The response is a sort of red herring which doesn't respond at all to the accusation but rather launches another in answer, thereby hoping to redirect the argument away from the original accusation.