Flash synchronization, or flash sync, makes sure the flash fires while the shutter is fully open, so the whole frame receives the light. Cameras with a focal-plane shutter can only do this up to a certain shutter speed, called the sync speed. It is often around 1/200 or 1/250 second. At faster speeds, the shutter is never fully open at once, and part of the frame stays dark.
Leaf shutters, built into some lenses and compact cameras, can sync at almost any speed. Many modern flashes also offer high-speed sync, which pulses the light to work at faster shutter speeds.
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