Shutter speed controls the amount of light entering your camera through its mirror — i.e., through the hole in the camera, as opposed to the lens.
DSLRs allow users to set the shutter speed from settings of around 1/4000th of a second through about 30 seconds … and on some models “Bulb,” which allows the photographer to keep the shutter open for as long as they choose.
Photographers use fast shutter speeds to freeze action, and they use slow shutter speeds at night to allow more light into the camera.
These are obviously just a couple of examples. However, slower shutter speeds mean that photographers won’t be able to hand hold their cameras and will need to use a tripod. It’s widely accepted that 1/60th of a second is the slowest speed at which it’s possible to hand hold.
So, a fast shutter speed only allows a small amount of light into the camera, while a slow shutter speed allows a lot of light into the camera.