The 18 - 55 is a zoom lens which has a focal length of 18mm at it's widest setting and 55mm at it's narrowest setting. In order to keep the cost of the lens lower, the designers allowed the physical constraints of the lens to allow more light to pass in the wide end than in the narrow or mild telephoto end. That is why the lens is rated from f3.5 to f5.6. This lens is much different from the old manual focus lenses you're familiar with in the past. It has a built in computer chip which communicates to the camera body. This is why the camera command wheel can tell the lens to adjust it's aperture. Every lens that Nikon makes has an aperture range up to f16, f22 or even f32. Lenses with macro capabilities will even go higher like f45 the closer you get to an object. It's just the laws of optics at work.
You have a great camera in the D5100 and a very sharp lens in your AFS 18-55mm.