The 80/20 Rule means that in any situation, 20 percent of the inputs or activities are responsible for 80 percent of the outcomes or results. In Pareto's case, it meant 20 percent of the people owned 80 percent of the wealth. In Juran's initial work applying the 80/20 rule to quality studies, he identified 20 percent of the defects causing 80 percent of the problems. Project Managers know that 20 percent of the work (the first 10 percent and the last 10 percent) consume 80 percent of the time and resources. There are a nearly unlimited number of examples that we tend to apply the 80/20 rule to in our personal and working lives. Most of the time, we are referencing Pareto's Rule without applying rigorous mathematical analysis to the situation.
We generalize about this 80/20 metric, but even with this sloppy math, the ratio is uncannily common in our world.