Word sync is a technique for synchronizing digital audio signals between high-end professional devices such as CD players, audio I/O cards etc. It allows all the components in the signal path to process the data and remain synchronized with each other.
In simplest terms, one drummer beating out a pattern 10110 is great, but if two or three drummers beat that same 10110 pattern, but do not all start at exactly the same point in time, or keep exactly the same tempo as they are doing it, the result is a very muddy message for other devices which might need to listen to all three at once (e.g. a digital audio capture card on a computer or other multi-track recording device).