What is the close relationship between Bruno and Maria?How is it significant to the story ?

I think that the relationship between Bruno and Maria is
significant in a couple of ways.  The first is that it represents one of the first
moments that Bruno's sense of compassion and understanding about the world is expanded. 
Once he understands more about Maria, and once their relationship develops into a
greater bond, Bruno advocates for someone whose experience lies outside his own. 
Additionally, the emotional proximity of their relationship is significant because it
compels Bruno to stand up for her when Gretel treats her callously.  It is in this where
Bruno shows himself to be an advocate for another person.  At the same time, as Bruno
develops greater feeling for her, he demonstrates more understanding and greater
awareness of her own predicament and her own narrative.  This is something that will
develop further in the novel as with Pavel and, of course, Shmuel.  It is significant
that Bruno's relationship with Maria is furthered and developed in a stronger capacity
because it will enable him to be able to defend someone who is in need, to represent a
universal quality in a world of contingent beliefs.