Complex sentence: If he should say "I will come out into the rain with you" it means that the new inmate is saying that he is prepared to live like him, they are brothers and they will live and die together.
Split into simpler sentences: If he should say "I will come out into the rain with you" it means "I will come out into the rain with you". This means that the new inmate is saying that he is prepared to live like him, they are brothers and they will live and die together.