Named by Italian Economist Vilfredo Pareto.
This analysis is also called 20/80 rule or Pareto Principle.
Basically we calculate how big part of first variable contributes to 80% of second variable.

Or more easily in this example – how big percentage of customers make 80% of sales or profit or something else.
If we find that 20% or less % fulfil this then this is very important information for marketing.