Also known as Pareto's Principle and a game-changer when it comes to long-term efficiency. A large percentage of effects in any large system are caused by a low percentage of variables.
The 80/20 rule pretends, that approx 80% of the effects generated by any large system are caused by 20% of the variables in that specific system.
The specific percentages are not important, they can vary between 10% and 30%. The thoughts behind are important: Pareto's principle is a very good tool for focusing resources and, in return, getting greater efficiencies in your design process. 80% of the results can be done in 20% of the total effort/time. Those affected 20% of the results with 80% of the work. Focus on the first one and you will boost your efficiency up. That means design and user-testing should primarily focus on those 20% effort features. The remaining 80% of the features should be reevaluated to verify the value in your design and you can think about if they are really needed and give a certain value. You can't only apply that to new designs or products, but also on existing ones where you want to increase the efficiency.
All elements in your design are not created equal. Use this rule to evaluate the value of elements, target areas of redesign or optimization, and focus resources in an efficient way.
Noncritical elements of your design should be minimized or removed altogether because they are part of the 80% less-important part. If you still have some resources left in the end, you can use these to work on the 20% effects, that need 80% of the time to realize them.