Why do I have a video inlet connection on my pc?

I have a realtek RTL8139/810x family fast ethernet NIC card in my pc that has a video outlet or inlet connection. My computer is a ACPI Uniprocessor (AMD Athlon 64 processor 3200+) with a display adapter by Nvidia (GeForce 8400 GS). the video inlet/outlet is an rca type outlet/inlet. what is this for and how do I use it? I've tried plugging my security camera receiver into it but nothing comes up on my pc screen and I don't know how to go about using it. any help would be appreciated. thanks