Throughout this play, Hamlet has contemplated death and what happens after death. He contemplated suicide, but worried about God's command against it. He contemplated suicide in general and realized that no one ever returns from the dead to tell us, the living, what it is like, so the fear of the unknown, keeps us all from killing ourselves as a relief from the burdens of life. By Act 5, he has managed to escape death by re-writing the letter to England, and he is now literally standing in a graveyard, talking to the grave digger whose whole life is about death. What he is asking in this quote is a rhetorical question that implies that Hamlet understands that death is the great equalizer and that death is kind of meaningless. He is alluding to the old expression that death means "ashes to ashes, dust to dust." That we come from dust and return to dust after death. No matter who we are, or what we did in life, it all doesn't matter after death. The lowly beggar and the great Julius Caesar will endure the same fate -- dust. He is saying that even the great Caesar is going to be dust that might be used to create a mud paste that will keep the wind out of a house. Once Hamlet realizes that death is kind of meaningless, it frees him to act and do what he needs to do in order to avenge his father's death and take care of Claudius once and for all.