those figures show the lens' maximum aperture across its zoom range, so in your case it would be f/3.5 at 18mm & f/5.6 at 55mm
those figures are the absolute maximum your lens can give you but at any time & at any zoom setting you can choose a smaller aperture than those numbers given, lets say up to a maximum of f/22 (fairly common size)
also you must remember that as aperture numbers go up
the size of the hole in the lens gets smaller (lets in less light)
have a play with the simulator here -
http://www.kamerasimulator.se/eng/?page_id=2
which will show you how different aperture,shutterspeed & ISO affect the image & how you can juggle all 3 to get the image you want
