Is the IP forwarding module present in the 2.2.* kernels.
An excerpt from IPchains' man page:
"Ipchains is used to set up, maintain, and inspect the IP firewall rules in the Linux kernel. (...)"
Efectively, what it does is analize each packet that passes by and determine what should be done with it.
It is usually used to turn a Linux station into a gateway between an intranet (or set of computers connected to each other, normaly through a hub) and the internet.
It's mainly usefull to set up a private firewall, since the gateway can control what goes in and out of the inner network.