what does the return of kino and juana and their throwing away the pearl mean to you?

It means that Kino understands the power the pearl holds over men's hearts. Juana understood this much earlier but could do nothing. The return of the pearl to the ocean, to me, is like throwing the ring into Mordor from Tolkien's Lord of the Rings. It is an acceptance of the evil that is ignited in the hearts of men from the pearl. It is a rejection of uncontrolled greed.