Port mirroring is a feature that allows a switch to copy the traffic from one or more ports to another port, where a network analyzer or a monitoring device can capture and analyze the traffic. Port mirroring is useful for troubleshooting and security purposes, as it allows the network engineer to see the traffic that is passing through the switch without affecting the normal operation of the network.
Reference
1: Port Mirroring - CompTIA Network+ Certification (N10-008): The Total Course [Video]
2: CompTIA Network+ Certification Exam Objectives, page 5
3: CompTIA Network+ N10-005: 2.1 - Port Mirroring - Professor Messer IT Certification Training Courses
4: CompTIA Network+ N10-005: 1.4 - Port Mirroring