What do all the numbers on the lens mean?

There are two different things the numbers on a lens means. The first thing is the aperture which controls how much light the lens allows in. The higher the number the lesser the light the lower the number, more light. This usually ranges from f4.0- f22.Which relates heavily with depth of field. The second thing is focus, it is very crucial to be super accurate with focus which is why the numbers are there. A lot of cheaper(consumer) lenses only have focus on them and the aperture is controlled internally from the camera. You'll find both aperture and focus on more expensive lenses which are called PL mount lenses which are the industry standard lenses.