When evaluating digital audio clocks, two attributes are often discussed - jitter and frequency drift. One attribute is crucial to the quality of your digital audio system, the other is certainly important but less critical.
To understand the difference between jitter and frequency drift, here are a few graphical representations of a word clock signal. Here's a good word clock signal compared to a jittery one - notice that the space between the vertical transition lines is consistent in the good clock, but inconsistent in the jittery clock.
Now, here's a good clock compared to one with whose frequency has drifted. Note that the space between vertical transition line is consistent in both examples, just smaller in one.
It's crucial that digital audio clocks have the lowest jitter possible, above all in the A-to-D and D-to-A conversion stages. To learn why, check out this KnowledgeBase entry: What is jitter?
It's also important that a clock's frequency is stable, but at a certain point an improved frequency stability offers no real benefit to a digital audio system. Features such as "atomic" or "oven-controlled" clocks provide a level of frequency stability that provides no audio benefit, just marketing talking points.
To get a better idea of the difference between jitter and frequency drift, think about the projection of a film. A film in which frame jitter occurs will result in jerky and unnatural movement - something immediately perceptible. A film in which frame frequency drifts may play for 2 hours, 10 minutes and 2 seconds instead of 2 hours, 10 minutes and 0 seconds - no one could ever perceive a difference.
Here's an analogy to understand this issue - imagine a home builder that advertised precision carpentry when framing your house. He might advertise that all 2x4s are measured with a digital micrometer and cut with a laser saw. This sounds like something you'd want, but would this result in a sturdier house? Probably not - it's a level of precision that's not necessary when framing a house. Worse, if a home builder is focused on his high tech process while ignoring the fundamentals of construction, it may turn out worse!