Someone you love and respect. A brother doesn't have to be a blood-related sibling.
Coming from a guys perspective in the Midwest, it is not often when a friend, relative or acquaintance calls me "brother", "bro" "brotha" or "broseph". Calling someone any of the those words is exactly like saying "I have positive feelings for you, and I love you, man." In a heterosexual, your-my friend-and-I-respect-you way.
For example, over the past year, the only time I called an acquaintance "brother, "bro" or "brotha" was when when I liked them, and thought of them as somebody pretty much love (for who they are)
Seth: "Happy birthday, brother."
Seth referred to Jared as "brother" because he loves him like a brother, even though they are not related.