A lens that is referred to by two numbers like that is a zoom lens (lens that can change it's focal length between a range of focal lengths. In this case, it is from 12mm to 24mm. The smaller the focal length, the wider the angle of the view at that focal length. Focal length is the distance from the optical centre of the lens to its focal point (where an object at infinity is in focus - usually the focal plane where the film is or sensor in a dSLR).
Why is this? Think of this analogy as it relates to angle of view. When you are looking out a window, the closer you are to the window, the greater the angle of view. If you (the focal plane) moves closer to the window (the optical centre of the lens), you are able to see more. As you adjust your distance from the window (focal length), you will be able to see more or less depending on whether you move closer to the window (shorten the focal length) or move further from the window (longer focal length).