The aperture is simply the hole in the lens that lets light through.
A small number such as f2.8, f/4 or f/5.6 will give you a physically larger aperture and the effect of this is twofold. It lets in more light for a faster shutter speed, but importantly gives a shallow depth of field. Using a large f/numbersuch as f/16 or f/22 will reduce the light and give you a larger depth of field. The lens itself dictates the range of apetures available. With a lens kit you can usually go from f/5.6 to f/22 when zoomed in.
