What Does Flexible Single-Master Operation (FSMO) Mean?
Flexible single master operation (FSMO) is a Microsoft Active Directory feature that is a specialized domain controller task used when standard data transfer and update methods are inadequate. Tasks that do not suit multimaster replication are only viable as flexible single-master operations.
Multimaster models have a number of operators, which are held by a single master. This problem is solved by applying a number of operations to a single domain controller. A single domain controller holds the role for a particular operation and is the single master for that operation. These operation masters are called flexible single-master operations.
Techopedia Explains Flexible Single-Master Operation (FSMO)
Domain controllers can hold one or more FSMO roles. These are functionalities related to the Windows 2000 Active directory service, which is unique within the domain. There are two forest-wide FSMO roles and three domain-wide FSMO roles. The number of FSMO roles in a forest depends on the number of domains in that forest. The forest-wide FSMO roles are schema master and domain naming master.
Schema master performs write operations to the directory schema. These schema updates are replicated from the schema master to other domain controllers in the forest. Schema master is normally hidden from everyday users because of the risk of the schema being corrupted by inexperienced administrators. It is revealed only by registering the schmgmt.dll file and adding the schema management snap-in to the Microsoft Management Console (MMC). An active directory forest has only one schema master. Domain name master adds or removes domains and cross-references objects to external directories. It also ensures that the child domain being added is unique.
FSMO roles can also be viewed, transferred and seized. They can be viewed using MMC tools or Visual Basic scripts. If a domain controller holds one or more FSMO roles, it's taken offline.