What is Fitts's law and how does it apply to button size and distance in UIs?

Fitts's law (simplified) states that the time to move to a target is related to the distance to it and the size of the target: larger and closer is faster. In interfaces, that means: make primary actions physically larger (within reason), and put them where the pointer or finger commonly rests or along natural motion paths. Screen edges in mouse UIs are special: a target on the very edge is effectively 'infinitely' tall, which is why corner menus in some OSs work. For touch, the law pushes you to generous targets and spacing. For data-hungry precision tasks (CAD), different rules; for everyday web and mobile apps, this is a standard interview line.

Frequent: large primary, near flow
Rare/destructive: smaller or farther, with confirm
# edge: full-height strip along dock / screen

Start simple: try this concept in a tiny project before moving to advanced tools.