A loopback plug is used to test serial ports, USB ports and network ports. Most commonly you will see network loopback plugs for fiber and ethernet. Loopback plugs reflect signals making it possible to verify that an electrical signal is successfully transmitting between two endpoints.
Loopback (also written loop-back) is the routing of electronic signals or digital data streams back to their source without intentional processing or modification. It is primarily a means of testing the communications infrastructure.