PRO = Performing Rights Organization.
A performance rights organization (PRO) provides intermediary functions, particularly collection of royalties, between copyright holders and parties who wish to use copyrighted works publicly in locations such as shopping and dining venues.
In the United States there are only 3 of these. ASCAP & BMI are open to anyone to join, while SESAC is for invited artists only.
Now these organizations will only get you part of the way there it seems towards all of your potential royalties. These are the royalties that can be collected from public performance. Clubs, Bars, etc. have to pay fees to these organizations every year. I am sure some of the smaller clubs slip by on this.
To Wikipedia for more…