Bubba means guy. "Hey, Bubba," is just a generic greeting that is used in some parts and is helpful (I guess) because you don't have to remember names. Everyone can be Bubba! It can also mean brother. I've heard some people call their brother that.
In the southern US, it started out as a nickname for brother, and mostly it was copying the infants' attempts to pronounce "brother" as "buh-buh" and it just caught on. Same thing kinda for "sister", which became "sissy". But "son" and "bubba" eventually became terms used for any youngish male.
Bubba - name for a close friend or boyfriend. Originating from Bubs. Bubba implies someone special and close to your heart. Bubba is someone you turn to when you need comfort. Your Bubba is always there for you.