The size of your aperture (the diameter of the hole through which light enters the camera) changes so that it can control the amount of light entering your lens. As stated it is measured in F-stops but these are counterintuitive, because the larger the number, the smaller the opening. Aperture is expressed as a fraction, measured in f-stops.
A low f-stop, or f-number, indicates a wider aperture, while a high f-stop narrower aperture. Just as ½ is a greater quantity than ¼, f/2 is wider than f/4. Both are wider than f/16. For example, f/2.8 allows twice as much light into the camera as f4, and 16 times as much light as f11.