He is dead and gone, lady,
He is dead and gone,
At his head a green-grass tuft;
At his heels a stone.

– William Shakespeare

Hamlet, Act 4, Scene 5. Ophelia sings about mourning the recent death of a loved one. The song is about the loss of her father Polonius, who played a central role in her life but was also a very controlling force. It is also about her loss of Hamlet, whom she loved. She is insane at this stage.