1) An awesome person who you have total respect for. It is used with lacrosse players a lot. They must have earned the right to be called a bro.
2) If used in a negative way it could mean a total douche bag. He is the annoying loud guy at a party. It is usually used in a sarcastic way like saying the person is cool.
1) Bob: He just did a back flip!
Joe: What a bro.
Bob: Hey man.
Joe: What up bro.
2) Fred: HEY EVERYONE IM AWESOME!
Charles: Wow, you are such a bro