‘Sync’ is short for ‘synchronization.’ When you’re talking about synchronization licensing in the music business, synchronization refers to the act of synchronizing a piece of music with any form of visual media—movies, TV episodes, commercials, video games, etc. Licensing refers to the act of getting the rights to use the music.
Music supervisors on film, TV, commercial, gaming and trailer projects must ask the rights holders of song(s) for permission to use the music in their projects. They have to get permission from all owners of a song: 1) The Master (or sound recording) owners, and 2) The “publishing” owners, which refers to the owners of the underlying musical composition. The music supervisors will ultimately need to obtain two licenses to use the song in their visual media project: 1) A “Master Use License” to use the master recording in coordination with the visual media, and 2) A “Synchronization License” to use the underlying musical composition in coordination with the visual media.