These are companies like ASCAP, BMI, SESAC, etc.
Performing Rights Organizations (PROs) are responsible for collecting income on behalf of songwriters and music publishers when a song is publicly broadcasted or publicly performed. Public Performances can include plays in television, radio, clubs, restaurants, websites, to live shows in various clubs, bars, and public spaces. PROs collect fees from these establishments which they then pay to their members, songwriters, and publishers.
Each territory worldwide has one or more PROs, responsible for collecting performance royalties for that specific territory. When you join a publishing administrator, they can register your songs directly with PROs around the world to collect your royalties directly from the territories in which they were generated. This gets you paid more quickly and with more detailed royalty statements.