Sync Speed is a term used to define the shutter speed while using a flash. Generally it is used to describe the fastest shutter speed your can use with your flash. This is usually in the range of 1/250s. If a faster shutter speed is used above the camera’s sync speed, then the whole image is not exposed by the flash burst. This is due to the flash duration being much shorter than the shutter speed.
A shutter will have 2 curtains that must open fully while the flash is fired, or else only a portion of the sensor is exposed to the flash burst. Newer cameras allow for faster sync speeds by firing mulitple bursts, but the usable distance of the flash is much shorter with this technique.