High-speed sync flash (HSS) is a technique in which the flash unit is able to sync at very short shutter speeds, such as 1/1000 second or faster. Normally, flash units can only sync up to a certain shutter speed (e.g. 1/200 second), but with HSS the flash unit is enabled to fire several rapid flashes in succession while the shutter moves. This allows you to shoot even in bright light or with a wide aperture, without underexposing parts of the photo. It makes it possible, for example, to blur the background while keeping the subject well-exposed, even in bright daylight.