as per the wiki page posted above, f stands for "
focal" ratio.
Wait till you hear about T stops! 
That's a cue then
The f/number - focal ratio - is as described above. Also known as the f-stop. By relating the focal length to the size of the aperture via a ratio, it means that f/4 passes the same amount of light regardless of the focal length - eg f/4 is f/4, whether it's on a phone cam or a 500mm super-tele.
It's only a theoretical measure of light transmission though, and it only applies accurately at infinity focus. Neither of these issues is a problem in normal use, but when you get to macro distances the effective amount of light passed reduces a lot. At 1:2 magnification ratio it's down to half, and to a quarter at 1:1. Of course, TTL metering takes account of this automatically.
Then there is the question of light absored by the glass, and lost through internal reflections inside the lens. This used to be a problem before anti-reflection coatings were so good, which led to the introduction of T-stops (transmission stops) marked on lenses. I've actually only ever seen them on old movie camera lenses, those huge multi-element zooms, which lost a lot of light internally.
TBH, there is still a small difference between f/numbers and T-stops on some lenses. If you compare a good prime with maybe six elements with a zoom having say 18 elements, then shoot at the same f/number, the zoom image will often be darker, but in the overall scheme of things (vignetting etc) and the fact that TTL metering compensates automatically, it's not a major issue.
that looks way too mathematical and scary to even contemplate trying to read and understand.
