The "flash sync speed" is how the camera synchronizes with the flash so that the light emitted by the flash is recorded on the imaging sensor.
The highest flash sync speed of any camera is the fastest shutter speed at which the entire sensor is exposed completely to the burst of light. For some DSLRs the flash sync speed is 1/200th second while others go up to 1/320th second.
Your camera’s shutter is like two curtains covering a window. When you trip the shutter, the first curtain opens to reveal the sensor to the light coming in through the lens, then the second curtain closes to cover it back up. The time that the curtains are open is the shutter speed. If you use a really fast shutter speed, like 1/4000 of a second, then the first curtain begins opening, and the second curtain begins closing almost immediately afterward, so there’s just a slit open between the two as they move across the sensor. When using available light (so-called Natural Light), it doesn’t matter what shutter speed you use because the light is always on and it comes through the lens even when there is only a slit to come through.
But when you use flash (or strobe), the flash is only on for an instant. If the curtain (shutter) is partly closed when the flash fires, then light is not shining on the part of the sensor that is covered and you get the black bar on part of the picture.
Your camera’s flash sync speed is the fastest shutter speed that exposes the whole sensor before the second curtain begins to close. The camera synchronizes with the flash so it lights up the whole sensor while the curtains are both open wide. Of course, if you use a slower shutter speed, then the sensor will be exposed longer than the minimum necessary to sync with the flash
Some flashes, like the speedlights or studio flashes are made to work with your camera and are capable of High-Speed Sync. ( HSS ) That allows them to pulse rapidly at high shutter speeds and send light through that little slit at every point across the sensor. There are two tradeoffs: these flashes are more expensive, and using these pulses of light reduces the overall brightness of the flash. You can shop for flashes capable of high-speed sync (HSS) that are compatible with your camera brand.
High Speed Sync (HSS)
HSS creates very consistent lighting across the entire image.
HSS works by continuously pulsing the flash at incredibly high speeds creating a stroboscopic effect that illuminates the shutter slit as it moves down the sensor. Because it has to output so many pulses of light, creating essentially a continuous light source for a brief period of time, the actual light output of the flash is quite low and can vary wildly depending on the shutter speed.
A flash in HSS acts much like a constant light (similar to controlling ambient light: the faster your shutter speed the darker the ambient light gets) and as such you will start to lose flash power at higher shutter speeds. It basically turns your flash into a high-speed strobe light. It pulses light extremely fast and consistent while your shutter curtains are passing in front of your sensor. This pulsing is not noticeable to the human eye.