[Sillzen]

It changes the focal length of the lens, although it does not seem to affect the lens (blue ring) in the left-hand pane which seems like a bug.

There is a demonstration later in the article (below the 5th mention of "focal length") which does affect the lens (blue ring) in the left-hand pane. Rotate the scene so that you are viewing the camera side-on, and you will see the focal length of the lens changing when you move the slider.