This is the fastness or slowness of the camera in recording the picture. If the shutter is so slow, much light gets into the camera sensor. If the shutter on the other hand is so fast, less light gets into the camera’s sensor. It is shutter speed which allows action shoot which is a situation where any motion picture is frozen and also allows waterfall which is a situation of blurring any motion. For freezing images, you use a fast speed of the shutter and for waterfall since the images need be blurred hence require more light in camera’s sensor, you use lower speed of the shutter.
Always remember, your camera can be held to capture anything which is falling above 1/50. And therefore in case your shutter speed is reduced below that, for the picture not to be blurry, you have to use something like a tripod. However, just like with ISO it’s varied in various cameras and therefore practice using your camera to get to know times when a tripod is needed.