Understanding the Pareto Principle can help you focus your time on the most important things.
The Pareto Principle (or the 80/20 rule) states that ~80% of effects come from ~20% of the causes.
You may have inadvertently encountered The Pareto Principle before in your life, which states that 80% of consequences come from 20% of the causes.
The principle is named after Vilfredo Pareto, an Italian economist, who noticed that approximately 80% of the land in Italy was owned by 20% of the population (hence the image of farm land at the top of the page...)
This principle is useful when helping to prioritise time. It could be found that 80% of the problems your users are facing could be solved by fixing 20% of the usability issues on your product, which helps your prioritise these. It could also be found that 80% of your companies revenue comes from 20% of your customers, helping you prioritise these people.
The Pareto Principle shows that the relationship between inputs and outputs is generally uneven.
The Pareto Principle is not a strict law, nor is it completely mathmatically accurate.
Other UX Laws
If a computer responds to a users input in less than 400ms, the user will take less time to make their next decision.
Hick’s Law states that the more options a user is presented with, the longer it will take them to make a decision.
Fitts's Law states that the amount of time taken to move to and select a target is a function of the distance to and size of the target.