Jitter is a time-based signal error. Often misunderstood, this problem can range from subtle and nearly inaudible to loud and distracting. Jitter can reduce low-level resolution (add noise) and add distortion.
What are they talking about when they say "Jitter"? Well, it can be several things. This is part of the reason people are often confused with the topic. Jitter can be induced by a poorly designed clock (the part of the digital device that sets the sample frequency. . . basically), from a digital interconnect that has a mismatched impedance, from a compact disc that has poor pit geometry (or poor laser focus), and/or from multiple digital sources with multiple clocks (improperly connected/configured) to name a few. Reducing jitter is most critical during analog to digital and digital to analog conversion. A large part of how well your AD or DA converter handles this is from its clock design.