PDC emulator is one of the five FSMO roles in an active directory domain controller. It is a domain-wide role and it has following major responsibilities:
1. It synchronizes the time across DCs in a domain. All DC member clocks are synched with the clock of the DC having the PDC emulator role. If there are multiple domains in the forest, then the forest root PDC is the authoritative time source for all domains.
2. It updates passwords for users and computers. When a user enters a wrong password to login, it's the DC with PDC emulator role that is consulted to check if the password has really been changed without partner DC knowledge.