What is the significance of the various skulls the gravedigger digs up during this scene? How do they contribute to the evolution of Hamlet's understanding of death?

In this scene from Hamlet Act V, the skulls are a symbolism; they represent the death (end of life on earth). They contribute to Hamlet's understanding of death because, as he sees the anonymous skulls, he begins to realize that nothing accomplished in life matters in the face of death, as we are all equal in death.