Shutter Speed
This is the amount of time that your shutter is open. When looking at your camera your shutter speed is written as 1/(a number). This means that your shutter is open for 1/(whatever the #) of a second. When shooting people and especially children try not to go any slower than 1/125. This will help to prevent a blurry picture. Sometimes 1/125 is not fast enough and you still get some movement in your photo. If your shutter speed gets too slow then “camera shake” may affect the sharpness of your photo. You shake your camera whether you mean to or not, which is why you want to keep your shutter speed as high as possible.
The lower the bottom number the more light will come in because your shutter is open longer. The higher the bottom number means less light will be coming in because it is open for less time. If you are in a low lighting situation and you are taking a picture of something stationary, you can lower your shutter speed to something crazy like 1/20 but just make sure you use a tripod!
Remember: Lower shutter speed = more light but your subject my be blurry / higher shutter speed = less light but possibly a sharper subject
(wording taken from this site)