Word Clock is the timing reference that keeps the sampling rate constant. If, for example, we were recording at CD quality which means the A/D converter has to take 44,100 measurements of the incoming sound wave every second. If the clock isn’t as stable as it should be, then some seconds will have 44,100 samples while others might have more or less. The end result is a less than perfect digital representation of the wave. The clock frequency is the same value as the sampling rate.