Many cameras feature a shutter setting marked "X Sync" or just "X". This is merely the fastest speed at which an electronic flash can be fired whilst the shutter is fully open. At speeds faster than the "X " setting, correct exposure is achieved progressively as a gap between the leading and trailing (first and second) shutter blinds moves across the frame. At speeds lower than the "X" setting, there is a period where the leading blind has fully opened but the trailing blind has not started to move. The entire frame can therefore be exposed at the same moment to light emitted by the flash.
The "X" sync speed varies from camera to camera. In high-quality cameras it is likely to be in the region of 1/300 sec although some offer flash synchronization at 1/500sec. In cheaper cameras it may be as slow as 1/60 sec. Typical speeds in mid-range cameras are 1/125sec, 1/200sec and 1/250sec.