Here is a close-up view of the Optical Inputs and Output.
These connections are used to access Dolby Digital or DTS surround sound from DVD players, Digital Cable Boxes, Satellite Boxes, or other source component that has a Digital Optical output.
It is also important to note that some CD-Only players have digital audio outputs as well, however, these are just two channel outputs.
You will also notice that there is one Digital Optical Audio Output. This allows a digital audio connection between the receiver to a CD Recorder or other recording device that has Digital Optical Inputs.