If the lens has a range, e.g. "18-270mm f/3.5-6.3", it means the maximum aperture varies depending on what focal length you're using. On the lens you describe, the maximum aperture is f/3.5 at 18mm, and f/6.3 at 270mm. As you zoom in (from the wide 18mm to telephoto 270mm) the maximum aperture will decrease.