A blanket license allows a user to engage in certain uses of a large number of works under preset terms, without individual negotiation.
In the copyright context, such a license addresses all of a defined group of copyrighted works. It “covers” all of the relevant works like a blanket. In this way, it makes it easier to negotiate for the use of a work by making it possible to only make a deal once rather than entering into many separate agreements.
Usually, such licenses are granted and managed by collective rights management groups, which control access to thousands, or even millions, of copyrighted works.