The exposure in your camera determines how much light is recorded in your camera. When there is plenty of light you need short exposure times. At nighttime, the time of exposure will increase dramatically.
Most digital cameras will adjust automatically, you should know though that some cameras will allow you to manually adjust the exposure time.
Playing with exposure times is a great way to experiment with your camera and to find out different results that you might get. For example, you can take the same picture of running water at different exposure times and you will notice a great difference on how the water appears in the final product. Remember to use a tripod if you plan to use a long exposure time to avoid a blurry image.