The yellow port on the back of your ASUS P4C800-E Deluxe motherboard is an S/PDIF port. It is used solely for digital audio. These ports are commonly found on motherboards, and act as a digital output port for audio. This means you can play digital audio such as Dolby Digital 5.1, or DTS tracks from a DVD for example through the S/PDIF port.
It will allow you to plug an appropriate coax cable into the port, and into an audio/video (AV) receiver. The AV receiver would then decode what it passed through the port (the audio track) and output it as intended from your source (DTS, DD5.1).
If you'd like to read up more on here, check out the Wikipedia page on S/PDIF.