Each lens has numbers inscribed on the filter ring that give us important information about how it will perform. The information will read something like 1:1.8 f=45mm

The number specified in mm is the focal length; the distance between the optical center of the lens and the film plane of the camera when the lens is focused at infinity. This gives us an idea of the angle of view (AOV) as shorter lengths give wider angles. Here the focal length is 45mm.

Lenses with a focal length of 50mm are called “normal” because they create images with roughly the same perspective and size relationship of objects that we see with our eyes.

The other set of numbers refer to the maximum aperture of the lens. 1:1.8 indicates that the maximum aperture is f1.8. This is useful as we can tell how fast the lens is

Interesting note:
Up until the 1970s Minolta used a useful two letter suffix on the front of their lenses to designate the number of elements and groups.
The first letter designated the number of groups: Q=4, P=5, H=6…
The second letter designated the number of elements: D=4, E=5, F=6…
From these letters we can instantly tell from the PF on the lens of the Minolta AL for example, that it has 6 elements in 5 groups.