The comment suggests that Hamlet is going to pretend to be insane in order to avoid raising suspicion while plotting to kill his uncle. However, in some interpretations of the play, Hamlet actually is mad and Horatio is a figment of his imagination. That theory supported by the fact that few characters actually speak to him directly, and the fact that he alone survives the tragic ending of the play. Regardless of the interpretation, Hamlet clearly intends to feign madness to get revenge on his uncle.