Look further down the FAQ, it shows links
The Xbox
Advanced AV Pack allows integration between the Xbox video game system and televisions or receivers that utilize optical digital audio input for Dolby Digital playback, as well as S-Video input.
Or in other words, its a cable with an adapter that converts the AV signal to optical.
Looks like this.

Available
here.
Or the
Xbox High Definition AV Pack connects the console to most TVs with interlaced component video inputs including high definition ready TVs that support 480i and either 480p, 720p or 1080i component video signals. Look for HDTV support on Xbox game packages. Movie playback is in 480i only.
This pack also supports digital or surround sound audio receivers. Sound system with Dolby Digital decoding and a fiber optic cable required to enable Dolby Digital sound. Sound system with Dolby Pro Logic decoding required to enable Dolby Surround sound. (Additional cables and sound systems sold separately.) High definition ready television required to enable high definition features.
Looks like this (and cost $19 ish)
Advanced AV pack manual