Its a digital remote control system that is used in many professional edit systems.
It controls deck functions and transfers Timecode (no audio or video signals).
Most higher-end decks (DVCAM, DVCPro, BetaCam, Digital Beta, et al) have RS-422 (9-pin) connectors for RS-422.
FCP can control decks via RS-422 with a “USB to Serial adapter” (the Mac commands RS-422 via USA).