Shutter speed.
Shutter speed is measured in seconds or fractions of seconds. To capture something fast you need a quick shutter speed otherwise the image would come out blurred. If the shutter is slow it creates an effect called "motion blur". Shutter speed catches movement. The bigger the denominator(bottom number) the faster the speed
From left to right;
Fast shutter speed, Middle shutter speed, Slow shutter speed.
Fast shutter speed, Middle shutter speed, Slow shutter speed.
This image has a slow shutter speed causing motion blur on the moving car.