Selecting a target with a pointer becomes harder as the target gets smaller or farther away.

Fitts's law models movement time approximately as

$$T=a+b\log_2!\left(1+\frac{D}{W}\right),$$

where $D$ is the distance to the target, $W$ is its effective width along the movement direction, and $a$ and $b$ are empirically fitted constants for the task and input device.

The value of the model is the relationship: larger targets and shorter movement distances are generally faster and easier to acquire.

Frequently used actions therefore benefit from adequate target size and placement. Tiny controls crowded together increase both acquisition time and the chance of selecting the wrong neighbor.

Screen edges can sometimes behave like unusually large targets because the pointer cannot overshoot them in the blocked direction.

Fitts's law gives interaction design a quantitative reason to treat target geometry as part of usability rather than only visual styling.