Ever wondered why the shutter speed is set to a certain time?
That certain time is called the Flash sync time. It is the shutter speed at which the shutter is fully open in order to fully expose the film.
Most SLR's have a vertical travel shutter. There are two blades to the shutter, called curtains. At a slow shutter speed the 1st curtain will drop down and fold up at the bottom of the opening, then when the time has expired, the 2nd curtain will drop and cover it up again. When the first curtain is down and the second curtain has not yet started to fall down, the shutter is fully opened. At that moment the flash is fired, resulting in an evenly exposed image.
If the shutter time gets faster and faster, the shutter is never fully opened at one time because the second curtain falls down before the first curtain reached the bottom. So the film or sensor gets exposed by a slit of light. That slit travels from top to bottom. The fastest time the shutter is fully opened is called the flash sync time, which is usually 1/125 or 1/250.
Problems arise when you use flash with a shutter time faster than the flash sync time. A flash emits a short burst of light. Shorter than the time the shutter slit can travel from up to down, resulting in only the upper part of the film or sensor get exposed correctly.
There are however cameras/flashes which can deliver a number of flashes in a row resulting in faster flash sync time.
Never be afraid of what you like. (Miles Davis)