NAC stands for Network Access Control or Network Admission Contol. As the name suggests, it is used to control the access of network or in other words it restricts the nodes/hosts to access the network resources. NAC is governed by set of security policies.
Ex:- If a organisation have Bring Your Own Device (BYOD) policy and a user comes with his/her own system and if that machine is not compliant with the security policies then it is not allowed to connect into the organisation's network, like if the system's antivirus is not updated.
It is a network solution that enables only compliant, authenticated and trusted endpoint devices and nodes to access network resources and infrastructure. It also monitors and controls their activity once they are on the network.
NAC is implemented to minimize the risk from internal threats.
Pls correct me if I am wrong.