In Act 1, Scene 2 of Hamlet, he is talking with Claudius and Gertrude, who are both trying to convince him to stop grieving his fathers death and move on with his life. At one point, Gertrude (Hamlet's mother) asks him why he seems to be mourning this particular death for such a long time, to which Hamlet responds by explaining that his behavior and clothing can only give them a glimpse of the grief he is experiencing inside himself.
Therefore, we could conclude by saying that Hamlet believed that his behavior didn't provide a fair reflection of how he felt inside, as it was much more intense than what it looked like.