What does the quote Seems madam Nay it is I know not seems mean from hamlet?

In Hamlet, the full quote reads: "Seems, madam! nay, it is; I know not 'seems.' Tis not alone my inky cloak, good mother,/ Nor customary suits of solemn black" (Act 1, Scene 2). In this passage, Hamlet is responding to his mother, Queen Gertrude, who has asked him why he is wearing such dark clothing. Hamlet replies that he is not simply "seeming" to be sad, but that he is genuinely grieving over the death of his father, King Hamlet. He goes on to say that he is not putting on a show of grief, but that his feelings are real and authentic.