Shutter speeds are typically measured in fractions of a second, when they are under a second. For example 1/4 means a quarter of a second, while 1/250 means one two-hundred-and-fiftieth of a second. Since shutter speed means the amount of time your camera shutter is open, the longer the time (ranges for different cameras) means that the picture will be brighter while the least time would mean that the picture taken will be dark.
However most photographers use shutter speed to capture and obtain different types of effects. For example, capturing an animal or person running, while the object is clear, blurring the surroundings. This is called freeze-motion and can be obtained by reducing the shutter speed. While increasing the shutter speed can capture the images like the motion of water while keeping the surroundings clear.