It's like "big brother," in both the literal and figurative meanings (well, not the 1984 figurative meaning...). I think lower ranking gang members refer to their seniors as 兄貴 as well, but it's not just limited to gangs (btw the boss is お頭 (おかしら)). I had a friend who was older than his other Japanese friends, and he would organize get-togethers and whatnot for them, and they would call him 兄貴 (although there may have been some humor involved in that). I've also heard it used as a term of address for one's older brother in the show 絶対彼氏, but not in real life that I can remember.