In photography, shutter speed or exposure time, is the length of time that the camera’s shutter is open and allowing light to hit the film plane or image sensor. Your camera’s shutter speed, the lens’ brightness (f-stop), and the luminance of the overall scene together will determine the amount of light that reaches your film plane (exposure).
There are multiple combinations of shutter speed and f-stop that will give the same exposure to a given scene. Doubling the exposure time will allow twice the amount of light in, as will making the f-stop one stop brighter. Below is a scale that might help you understand this better. In this chart (and all shutter speeds) the values have an assumed “1" over them unless they are followed by an “S". The first value, 250, is actually 1/250th of a second. The value “1S" means one second. This is a photographic standard.
Along with controlling the amount of light entering a camera, the shutter speed also controls motion and movement in your photography. It is recommended that when applying a shutter speed of great than 1/30th of a second, that the camera is placed on a tripod to avoid camera shake. This will allow you to “freeze" the scene that is not in motion but show movement to the subjects that are in motion (like car headlights or a person running). Here is an example of this:
On the contrary, a very high shutter speed will freeze everything in the scene that you are capturing. A high shutter speed will typically be used in sports photography such as racing or in the studio to freeze subjects in motion. Here is an example of a high shutter speed:
Hopefully this help you have a better understanding of what the shutter speed does, and how it works with the aperture and a scene’s luminance. Try adjusting your shutter speed the next time you are out shooting. Bring a tripod and start with your camera’s “Shutter Priority" feature. Try some long exposures, like 15 seconds and up, and see what happens to the light and motion in your images.