A master license gives the license holder the right to use a recorded piece of music in a media project, often a film, TV show, commercial or some other visual creation, though the license does extend to audio projects as well. A master license is obtained from the person who owns the recording (in other words, owns the master). Often master licenses are obtained from record labels.
Note that a master license differs from a sync license in that a sync license gives the license holder the right to re-record a song for use in a media project, while a master license gives the license holder the right to use a pre-existing recording of that song in a media project. A sync license (from the publisher) is usually required in conjunction with a master license (from the master owner) to use a pre-existing version of a song with a visual project.
A master license can only be issued for one song at a time. Even if someone wishes to use an entire album in a project, a license must be obtained for each song individually.