There are only two companies that control the majority of the radio airwaves: iHeartMedia (formerly Clear Channel) and Sirius XM. They work in tangent with performance rights royalty collections agencies like ASCAP and BMI to ensure plays are tracked and artists are paid.
They do this by both digital monitoring and requiring partner stations to submit tracked playlists.
A playlist is a list of songs that a radio station plans to play on a regular basis. The same term is used by MTV, VH1 and other music television channels for videos they play regularly. Radio stations that work with playlists usually have a tiered listing system – songs that are on the A list get played X number of times a day, the B list slightly fewer times, and the C list fewer again.
In most cases, DJs are bound by the playlist and obligated to meet the requirements of the list before they can select music themselves to play – in fact, many radio shows are determined entirely by the playlist or the DJ may get as a few as one or two “free plays” that they can use to play songs they think are great. If you hear that a song is in “heavy rotation,” it means that it is high on the playlist.
These kinds of playlists are typically determined in playlist meetings, during which radio pluggers meet with reps from the radio station, play the songs they are promoting and try to convince the station to put the songs into rotation.