High-speed sync
You might wonder why ‘flash sync speed’ is a thing. After all, a burst of flash is lightning-quick, so why can’t it be used at fast shutter speeds? It’s down to the way that the pair of shutter curtains in front of the sensor work. At the sync speed or slower, both curtains are open and the whole surface of the sensor is exposed when you take a picture, but at faster shutter speeds the sensor isn’t fully exposed in one go – the second curtain starts to close before the first one has fully opened, meaning that the sensor is essentially exposed through a fast-moving slit; if the flash fired, its brief burst of light would only register on the part of the sensor exposed at that time.
There is a solution: high-speed synchronization (HSS). Available in many flashguns, it fires a sequence of low-power flashes to coincide with the moving slit created by the shutter curtains, so you’re free to use large apertures (and the corresponding fast shutter speeds) to blur backgrounds.