the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target.
The most common application of this law these days can be paraphrased as “The wider an object is on the screen and the closer it is to the mouse pointer the easier it is for the user to acquire that target object.”
Maximising this feature for the most important objects in a UI can be crucial to making that UI usable. For example, it’s arguably one of the main reasons why macOS in general tends to fare better in terms of usability than Windows: While macOS’ menubar is attached to the top of the screen no matter where the currently active application window is positioned and therefore has infinite width its Windows equivalent is attached to the current window instead and therefore is much more difficult to acquire.
This infinite width (or infinite edge) phenomenon also imposes interesting constraints on web application design because web applications typically reside within a browser application window and hence cannot profit from such an infinite edge.
In his article on Visualizing Fitt’s Law designer Kevin Hale does a great job at both visualising and explaining the “math of the obvious” behind Fitt’s Law.