NAC (short for Network Access Control) is a security system that controls device access and traffic on your internal network. When a computer connects to a network (via wireless or wired means), it is scanned by the NAC for software and update requirements. These requirements, known as a policy, can include things like anti-virus protection levels, system update levels, configurations, and firewall settings.
If the device being checked isn’t compliant with the policy there are several options that the NAC can be set to enforce including not allowing it on the network. The NAC continues to control what a user is allowed to access even after they have been authenticated and authorized, providing an added level of security.