What is it?

Probably one of the most common known and universally applied principles.
It says:

A high percentage of effects in any large system are caused by a low percentage of variables

The 80/20 rule was discovered by Vilfredo Pareto, an Italian economist, who observed that 80% of the wealth in Italy was possessed by only 20% of the population.

The Pareto Principle can be applied to any large system.

While the exact percentage is not important, it describes a universal phenomenon of unequal distribution of value.

Why is it important?

Knowing this law of distribution helps to focus energy and resources on the minority of elements of a design that creates the majority of the value for users.

When resources are scarce, reducing efforts to correct and optimize elements that do not contribute to the overall success of a product, can be of crucial importance for the survival of a business.