What did Rose’s Uncle mean when he says “a child needs walls . . . walls to keep them in and others out”?

Jonas was quiet but told Rose that their family survived residential schools and cautioned her to trust no one. He'd say that children need walls of some sort to keep them safe. He declared that he was Rose's walls, and when he died, that Rose should use a rifle to make her own walls. This makes sense in a world that is less than kind. It is a way of protecting a person from trauma.