802.1Q tagging allows switches to insert a VLAN tag in the Ethernet frames, specifying the VLAN to which the frame belongs. This tagging enables the frame to be correctly identified and handled by other devices in the network, maintaining traffic segregation between different VLANs on the same network link. Port mirroring, also known as SPAN, does not involve traffic segregation but is used for monitoring purposes. Port security restricts the input to an interface based on MAC address limits, and MTU settings involve the largest packet size that can be sent over a network, neither of which are directly related to VLAN tagging and traffic segregation.