Jitter is the timing variation in the sample rate clock of the digital process. It would be wonderful to believe that a sample rate of 44.1 kHz is an exact science, whereby the process samples at exactly 44,100 cycles per second. Unfortunately, this isn’t always the case. The speed at which this process takes place usually falters and varies and we get the ‘wobbling’ of the clock trying to keep up with the speeds of this process at these frequencies. This is called jitter. Jitter can cause all sorts of problems and it is best explained, for you, as: the lower the jitter the better the audio representation. This is sometimes why we use better clocks and slave our sound cards to these clocks, to eradicate or diminish ‘jitter’ and the effects caused by it.
Jitter is a variation in the timing of the sampling instants (time based) when the audio is converted to or from the digital domain. If the conversion process suffers from any time anomaly then the resulting signal amplitude will differ from its true value. Usual side effects are an increase in high frequency noise, clicks and worst case scenario muted and not working. In simple terms, the clicks are caused when one of the digital devices searches for an incoming audio 'sample' but fails to find it as it is looking at the wrong time 'frame' (instance). Apart from these 'anomalies' the real world audio effect is that the stereo imaging is compromised leading to a flat stereo image as opposed to one with depth and width.
Jitter affects the stability of the sample clock. The lower the jitter figure, the more stable the clock and the better the performance. This means that the lower the jitter values, the better the performance and the more stable the clock is.
When using more than one digital device it is best to interface and synchronize, using clock synchronization, between both the source and destination digital device.
Most of today's digital systems will have embedded clock at source that can then be used to synchronize the two devices.In more sophisticated systems like DAWs, digital consoles, higher end sound cards and so on, there will be some form of control panel whereby desired clock sources can be selected. The most common selections available are: digital input, external word clock, and internal clock. Selection comes down to system configuration and project choice. However, what is a given is that all digital devices must be synchronized.
Using the internal clock ensures stability as the clock rate is known, but this is where all devices must be synchronized to the internal clock's rate. Alternatively, and a common choice amongst most studios, is to use a dedicated external clock. This affords a universal and global rate that all devices can be synchronized to, and more importantly, a dedicated master clock has one function and that can often alleviate system configuration problems. The only problem that arises from this scenario is that most consumer systems do not accommodate for slaving to external clocks and the internal clock will have to be the master clock source.
At the end of the day, it comes down to knowledge and experience and ignoring the benefits of a good clock source in a digitally configured system is the equivalent of running top end processors through a Radio Shack budget 2 channel DJ mixer.