Probably easiest to understand by example.
Take two function generators.
Set both to 1MHz exactly.
If you look at the output on a scope you'll almost certainly see one waveform move relative to the other.
This is because both generators have slightly different references.
If you essentially remove one of the references, and have both generators use the same signal for frequency reference, that stops happening, and this is exactly what's done in synchronous networks (which, outside cellular networks, and some very specific scientific use cases, are obsolete), as well as many RF labs.