It seems for the sake of space, time, and effort, surfers shortened "brother" to "bro" and "brah", therfore creating a concise pronoun to mean friend.
Surfers have an almost endless supply of terms for friend or cohort which is, I think, symbolic of our innate brother and sisterhood. "Brah" is one of these words; however, like the word "dude", it may take on other connotations in different contexts.
Also Known As: bro, brother, brudda, broham, my man...The list is endless.
(Friend) Hey brah, we have got to take a trip to Costa this summer.
(Not a friend) Ho brah, paddle in and go back to the mainland already before you get doughnuts(black eyes).