A domain controller (DC) is a server that handles all the security requests from other computers and servers within the Windows Server domain.

there was a primary domain controller and a backup domain controller. The primary DC focused on domain services only to avoid the possibility of a system slow down or crash due to overtasking from managing other functionality and security requests. In the event of a primary DC going down, a backup DC could be promoted and become the primary DC to keep the rest of the server systems functioning correctly.