PRO stands for Performing Rights Organization, and their main purpose is to represent songwriters and their music works. They license your music out to businesses, granting businesses permission to legally play your music in public. Radio broadcasters and club establishments must also acquire a license from PRO's before playing your music. Technically, you could go directly to radio stations yourself and license your music directly, therefore obtaining all the proceeds your music generates, however you'd have to gain a relationship with each and every radio station; which is still possible.
PRO's usually issue blanket licenses which give businesses access to play from a whole catalogue of music. Each air-play of your music is paid royalties, collected by your collection society (Your PRO) for the use of your music, which then are allocated back to you as the rights holder. This is what is known as 'Performance Royalties' Businesses are required to pay this blanket license for a certain period of time, allowing your music to generate royalties. You receive performance royalties not only from live performances at events and gigs you attend, but also when your songs has broadcast on radio AM/FM, or played at a venue like a bar or club, any business establishment such as a gym or barber, etc., these are all classed as public performances. A stream-play on Spotify or YouTube are also regarded as a performance of your music, so a generated royalty must be paid out to you.