o, "optical in/out" describes a pair of small sockets on a recording device. They send and receive light rather than electricity to convey a digital bitstream (a digital audio signal, in other words). So they're a kind of "digital input and output" for the unit.
Using the optical input you can send a digital bitstream into the recorder (as an alternative to, say, connecting a microphone directly to the recorder's mike input); the optical output provides a digital bitstream that you could use for playback, or for "looping through" the recorder (i.e. getting an output that you use for something else at the same time as you're recording through the input).
If you intend to use the mike preamp built into the recorder, or an external mike preamp that doesn't have a built-in analog-to-digital converter, then the input may not interest you so much. The output may not interest you very much either, when you consider that it only delivers the audio "in real time"--if you hope to transfer a digital recording (say) to a computer for editing or other processing, you will almost certainly want a faster way to do that, such as USB or FireWire--or just taking the memory card out of the recorder and putting it into a card reader on your computer.
Does that answer your question, or was there more to it?
--best regards