Flash sync speed is the fastest shutter speed at which a camera's shutter is fully open at the moment a flash fires, typically around 1/200s-1/250s on most cameras.
Flash sync speed is the fastest shutter speed at which the sensor is ever fully uncovered in one instant. Go faster and the second curtain begins closing before the first has fully opened, so the flash burst only lights the moving slit and leaves a hard black band across part of the frame. This is the wall photographers hit when they try to mix flash with bright sun.
Balancing a backlit portrait at f/2.8 outdoors at midday might call for 1/1600s, but a camera that syncs at 1/200s will not let the flash fill the shadows at that speed without banding. The workarounds all cost something: drop to 1/200s and stack ND filters to hold f/2.8, switch to high-speed sync and give up two to three stops of flash power, or use a leaf-shutter lens that syncs at any speed. Knowing the number decides which of those you plan around.
A photographer is shooting a skateboarder against a bright sky and wants the flash to lift the shadows on the rider's face. At ISO 100, f/4, the sky meters at 1/2000s. Fire a standard hotshoe flash at 1/2000s on a body with a 1/200s sync speed and roughly the bottom two-thirds of every frame comes out black, where the shutter curtain covered the sensor during the flash pulse.
Switching the flash to high-speed sync clears the band, because it now pulses thousands of times across the exposure instead of once, but its effective output drops by about two and a half stops, so the flash has to move from three metres to under two to still register. The other route is 1/200s with a 3-stop ND filter, which keeps full flash power at the cost of carrying the filter.
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