The shutter speed is amount of time the film is exposed to incoming light. The slower the shutter speed, the more time the film is exposed to light. A photographer would want to use a slower shutter speed if he/she was taking a photo in a dark location, so the light has more time to enter, and vise versa.
A higher (or faster) shutter speed allows less light to hit the camera sensor or film strip (if using an analog camera). Conversely, a lower (or slower) shutter speed allows more light to pass into your camera. The focal length of your camera's lens can help you determine a base shutter speed.
