The Aperture Ring controls a set of metal leaves that can pivot creating a hole that can be larger or smaller, letting more or less light enter the camera lens. The settings on this ring are called F-stops. Most lenses have the F-stop settings of 2.8, 4, 5.6, 8, 11,16, and 22.
These settings represent the size of the adjustable hole. At F2.8, the size of the hole is really big. The hole gets smaller and smaller each setting going toward F22, which for most camera lenses is the smallest the hole can be made.