From what I have read it stands for Ultimate Bulletin Board (software).
It seems to be like an address book where you add people who you want to contact more often (PM or email) without looking through the user list which holds everyone.
Guess the explanation below is the best fit
Gabriele
"You can add anyone as a UBB Buddy. It's not a huge feature, but when you are sending PM's you can select one of your buddies from the drop down list. This is particularly helpful if your buddy has a similar username to someone else, or if they have special characters or spaces in their screen names." 
