It's the ratio of focal length to, effectively,the front element diameter. It has no dimension but, theoretically, provides a fixed reference of one lens to another. It does not take account of dispersion etc though and so one lens with a specific aperture will often vary from another due to differences in dispersion etc. However, this difference is usually very small and therefore negligible. In cinematography, the actually amount of light transmitted is accounted for / measured and the aperture values (often not click 'stopped') are shown as T (transmission) values in the same numeric progression.