Electronic-flash synchronization, as opposed to bulb-flash synchronization.
In electronic flash photography, the light discharge is very rapid, so the camera shutter must be fully open before the flash unit is triggered.
In cameras with a focal-plane shutter (a shutter that lies right in front of the film) the shutter does not open fully when high shutter speeds are used. Instead, the second-curtain of the shutter is released before the first-curtain has finished travelling, and the film is exposed by a slit passing across it.
Consequently, X-synchronization is not possible above a certain speed. The highest speed at which electronic flash synchronization is possible for a given camera is called the 'X-sync. speed'.
Synchronization is, of course, possible at slower speeds, but the effect of ambient light becomes more and more important as the speed is reduced, and at some point, the exposure due to ambient light may exceed that obtained from the flash.
In modern camera systems, a 'ready' signal from the flash is used to set the camera to the X-sync. speed if the user tries to select a higher speed.