Fitts’ law is one of the most important law for aiding user interface design. It provides a tool for predicting the time required to reach a target, given the size of the target and the distance to it.
Basically it describes the bigger and closer a target is, the faster you can grab it. That being said, the time required to reach a target is proportional to the size of the target, and inversely proportional to the distance to the target. Fitts law is widely applied in user interface design.