Radio pluggers specialize in promoting music to radio stations. They are hired by record labels to take new releases (usually singles, but in certain cases, full albums) to radio show producers and DJs to try and secure radio plays for that album. Radio pluggers try to get the releases they are representing put on playlists, but if they can't get them on a regular rotation playlist, they try to at least score a few plays. Radio pluggers may also try to set up radio sessions (during which a band either plays live or pre-records a live performance for use on a radio show) or interviews with the bands.
Most radio pluggers work on a campaign basis - that is, they work on a specific release for a pre-determined period of time. Further, some pluggers have a more specific focus, such as specializing in college radio.
You can learn more about playlists and other kinds of PR here: