A sync license (synchronization license) is an agreement that allows music to be synchronized with visual elements on a screen, typically in movies, TV advertisements, or video games, even though sync licenses are also needed for some audio uses, such as radio advertisements.
For a song to be synced, the licensee must require a license from the recording owner (artist/label) and the composition owner (songwriter/publisher). These licenses are usually paid as a one-time upfront fee, 50/50, between the recording and composition rightsholders.
Next to the upfront payment, songwriters and publishers earn performance royalties whenever the visual content containing the licensed song is broadcasted on TV.