A blanket license is type of license issued by a performing rights society allowing a music user to play or perform all compositions controlled by all publishers represented by that society.
The user will generally pay a yearly fee that allows them to use all licensed songs without limit. Blanket licenses are typically issued to nightclubs, TV networks, and radio stations.
Music publishers sometimes enter into "blanket licenses" with specific outlets with respect to their catalogs. For example, a publisher might give a television production company a "blanket license" to utilize any song in their catalog (or a limited list of songs) for a previously agreed upon rate. This would be in lieu of securing an individual sync license for each use.