Lenses have two types of MMs: diameter and focal length.
Let's look at each one.
The front of a lens has a diameter, such as 67mm.
You need to know the diameter of the lens if you're buying a filter.
The easiest way to determine the diameter is to look at the inside surface of the lens cap.
The diameter is usually printed there.
Technically, focal length is the optical distance from the front of the lens to the sensor.
Practically, focal length is how much you can see of a scene.
A wide-angle lens "sees" a lot from side-to-side.
18mm is a wide-angle focal length.
The number is below 50mm, the arbitrary point between wide and telephoto focal lengths.
A telephoto lens magnifies a distant part of a scene and brings it closer.
100mm is a telephoto focal length.
The number is larger than 50mm.
Some lenses have only a single focal length, a fixed focal length.
Many photographers have a 50mm lens because the lens lets in lots of light, and has less depth-of-field.
The lens only has one focal length, 50mm.
In contrast, zoom lenses have many focal lengths.
For example, an 18mm to 200mm zoom lens has all of the focal lengths between 18mm to 200mm.