Fitts's Law (pronounced "fitzez law") simply states the time to acquire a target is a function of the distance to and size of the target.
Fitts's Law applies mainly to user interface design. The simplest button to click is the closest and largest one. As an example of the first principle, right click menus are very efficient, because the distance between the target and the cursor is zero.
The latter principle is demonstrated by windowed applications. A windowed application has a buffer zone between the left toolbar and the edge of the screen. This buffer is the window's trim. When the user moves their mouse to click a tool, they are forced to slow down so they will not overshoot their target. By making the application full screen, the tool buttons will increase their size to include the edges of the screen. The user can then shoot their mouse to the corners and click without slowing down. Since the mouse will not run past the screen, the buttons have an effectively infinite area to the left.
Fitts's Law is generally one of the most important and ignored concepts of GUI design.