This line is spoken by Marcellus in Act I, scene iv (67)
"Something is rotten in the state of Denmark."
In this scene Hamlet and Marcelius are having a debate. This debate is about whether to follow the ghost or not. In general, this quote mentions that the ghost is a bad indication of things to come to Denmark hence " Something is rotten in the state of Denmark." To me the Ghost represents corruption in Denmark. Later in the Story we are told that the ghost is a representation of king Hamlets Murder (Corruption).