Zoom lenses with fixed MAX aperture are e.g. Tokina 12-24/4, Canon 70-200/4 etc. In each of these lenses you can open the lens to f/4 (not more) or close it to f/5.6, f/8, f/11 etc.
Lenses with fixed aperture are mirrors e.g. Tamron 500/8, Sigma 600/8 etc. In these lenses you can NOT change the aperture at all.