Port mirroring is used on a network switch to send a copy of network packets seen on one switch port (or an entire VLAN) to a network monitoring connection on another switch port.
This is commonly used for network appliances that require monitoring of network traffic such as an intrusion detection system, passive probe or real user monitoring (RUM) technology that is used to support application performance management (APM).
Other vendors have different names for it, such as Roving Analysis Port (RAP) on 3Com switches.
It helps administrators keep a close eye on network performance and alerts them when problems occur.
It can be used to mirror either inbound or outbound traffic (or both) on single or multiple interfaces.