The shutter is a small “curtain” in the camera that quickly rolls over the image sensor (the digital version of film) and allows light to shine onto the imaging sensor for a fraction of a second.
The longer the shutter allows light to shine onto the image sensor, the brighter the picture since more light is gathered.
A darker picture is produced when the shutter moves very quickly and only allows light to touch the imaging sensor for a tiny fraction of a second.
The duration that the shutter allows light onto the image sensor is called the Shutter Speed and is measured in fractions of a second.
A shutter speed of 1/2 of a second will allow more light to touch the image sensor and will produce a brighter picture than a shutter speed of 1/200 of a second.
So if you’re taking a picture and it is too dark, you could use a slower shutter speed to allow the camera to gather more light.
Just as the Aperture affects the exposure as well as the depth-of-field, the shutter affects more than just the exposure. The Shutter Speed is also principally responsible for controlling the amount of blur in a picture.
At a faster shutter speed, you can completely freeze the action of a moving subject. Conversely, when you use a slower shutter speed, you can blur the subject in the direction of motion, and therefore capture the motion of subjects such as flowing water.