High-speed sync flash is the camera's ability to use a flash at shutter speeds faster than the camera’s native sync.

Most cameras have a native sync of 1/250th of a second, and anything faster than that is beyond the camera’s ability to sync the shutter with the flash. But if you happen to be in a situation that requires faster shutter speeds to effectively capture the action, or for other aesthetic reasons (like a wide aperture), then you’ll over-exposure your image.

However, high-speed sync flash/camera combinations allow you to use the flash at higher shutter speeds.