The time it takes to click or tap a target is determined by two things: how large the target is and how far away it is. Bigger and closer is always faster.
Fitts' Law was published in 1954 by psychologist Paul Fitts, who was studying aircraft cockpit controls. The core finding: the time to acquire a target with a pointing device is a function of the ratio of the distance to the target divided by the width of the target. In plain terms — double the target size and pointing gets faster; double the distance and it gets slower.
This has immediate, concrete implications for interface design. Primary actions should be large. A submit button that's 44×44px is not just an accessibility guideline (it is, in both Apple and Google's HIG) — it's a physics rule. Users reach it faster and miss it less often.
Fitts' Law also has a lesser-known corollary about edges and corners. Because the screen is bounded, a button placed at the very edge of the screen is effectively infinitely large in one dimension — the cursor stops at the edge, so you can move your mouse as far as you like and it won't overshoot. This is why the macOS menu bar lives at the very top of the screen and why the Apple menu has been in the top-left corner since 1984. You can slam the mouse up and left and always land in it.
For touch interfaces, the law applies with a different parameter: the finger's contact patch is larger and less precise than a cursor, which is why mobile tap targets need to be larger still — 48dp in Material Design, 44pt in Human Interface Guidelines — even though fingers don't travel across a screen the way a mouse does. The "distance" on mobile is mostly about reach from a grip point.
Fitts' Law is often violated in small ways that add up: icon-only nav bars with 20px hit areas, dropdown menu items with 4px padding, a "Cancel" link in small text next to a full-width "Confirm" button. Each of those costs users fractions of a second. Across a product, those fractions add up.