A master policy is the insurance a condo or homeowners association carries on the building structure and common areas (roof, exterior walls, hallways, shared amenities). It is paid for through association dues and is separate from a unit owner's own condo policy (an HO-6), which covers the unit interior and belongings.
The dividing line between what the master policy covers and what the unit owner must insure is set in the association's governing documents, so it varies by building.
Example: a burst pipe damages both the building's shared wall and a unit's interior cabinets. The master policy handles the shared structure; the owner's HO-6 handles the interior.
Related: loss assessment, dwelling coverage, personal property coverage. See the condo insurance overview.