Operations master roles (also known as flexible single master operations or FSMO) are configured by default when Active Directory ...

Operations master roles (also known as flexible single master operations or FSMO) are configured by default when Active Directory Domain Services (AD DS) is installed. Typically, these roles do not require ongoing management. However, for changes such as a system state restore or replacement of a domain controller, you may need to transfer the operations master roles to another domain controller. As a last resort in case an operations master role cannot be transferred, the role can be seized to another domain controller.