What is the back door of the router?

Q: Yesterday saw a lot of news said, D-link Cisco and other manufacturers of routers exist back door, was hacked, what is the router back door? How could there be a back door?

A: The router back door is in the development of software, in order to detect debugging in the future to facilitate the operation of technicians and reserved "direct control authority." If the hacker cracked the back door, then you can get administrative privileges, you can remotely control the router, that is, we often see the router was "hijacked"