P.R.O. stands for Performing Rights Organization. The PROs most familiar to musicians are ASCAP, BMI & SESAC. Performing Rights Organizations collect royalties on behalf of their members and distribute those royalties based upon the amount those individual songs are performed on a quarterly basis (every three months).
"Performed" for these purposes not only means played live in venues, but also played on the radio, or synced to visual mediums i.e. used in a TV show.
It is always good to register your songs with whichever PRO you join, but you can only expect to receive royalties if your song is being "Performed". And even if your songs are being performed, you won't see the money for several billing cycles after they were performed.
An important point to note is that these royalties only go to the song writer and publisher.
So if you recorded a cover that is getting played a lot, you are out of luck.
You can however get a cover song synced to TV and receive a fee for use of the master recording even though you didn't write the song.
Where do the PROs collect money from? Any venue where music is performed, whether live, on the jukebox, by satellite, radio station or in a bar or restaurant, at the stadium etc, the venue has to pay for a license from the PROs which is then divided according to song usage.
One more important point to note is that a songwriter may only belong to one PRO but publishers may have separate companie for each PROs since they may represent songwriters with different PROs.
Check our links page for direct links to some of the different PROs.