A Pareto chart is a powerful visual tool that can help you understand the 80/20 rule, also known as the Pareto principle. This principle states that 80% of the effects come from 20% of the causes.
From Wikipedia
A Pareto chart is a type of chart that contains both bars and a line graph, where individual values are represented in descending order by bars, and the cumulative total is represented by the line. The chart is named for the Pareto principle, which, in turn, derives its name from Vilfredo Pareto, a noted Italian economist. Its purpose is to assess the most frequently occurring defects by category.