What does it mean when a switch restricts network segment access

Port-based network access control (PNAC) blocks access to a network by denying communication between an unauthorized device and the network. When an unauthorized device attempts to connect to a network port, the switch or router that manages the port will deny access to the network. NAC verifies who and what is connecting, checks whether a device meets security. Segmentation is the practice of dividing a network into smaller parts called segments to improve security, performance, manageability, and compliance. Of organizations say segmentation is a priority. Enabling this feature keeps track of permitted source MAC addresses and restricts the number of MACs that can use a specific port. [pdf]