What is domain controller?

A domain controller (DC) is a key server in the active directory infrastructure that manages authentication for the organisation’s users, as well as security policies.

This is typically on Windows Server systems, but this role can also be run on Linux systems. In addition to handling authentication, they usually perform other functions such as certificate management, RADIUS server, DNS server…
In an enterprise infrastructure, the domain controller is one of the most critical elements, so performing security audits to evaluate its security is fundamental.