Remember that no audio signal passes thru fiber optic cables; all data is represented by ones and zeroes. However, all data is serial and like all things digital, dependent on the time it is sent and received, hence the need for synchronizing digital devices.
Cheap fiber optic cables are prone to internal reflections which can cause the receiving digital device to receive the same 'bit' several times, usually after the first original bit has been received. This can be called echoes but unlike distinct reflections, think of it more as a 'blurring' of the original image.
What does jitter 'sound' like? Actually, jitter has no sound of it's own, being dependent on the program material. However, people have observed that when comparing a jittered signal versus the same material with less jitter, testers have stated that they hear a better soundstage, better depth and clarity, better stereo imaging, and better high frequency reproduction, among other things. A more in-depth analysis of jitter can be found
here.
Going from a consumer soundcard to a more pro soundcard with better clocks, you will immediately hear the difference in terms of better clarity; your ears strain less with a low jitter signal.
