Italian economist Vilfredo Pareto observed at the beginning of the 20th century that 80% of wealth is held by 20% of the population. But he also noticed that 20% of workers contribute 80% of the work, 20% of criminals commit 80% of crimes, and today we know that 20% of drivers cause 80% of accidents, 20% of hedge funds own 80% of funds, 20% of drinkers consume 80% of drinks, we wear 20% of the clothes in our closet and spend 80% of our time with 20% of our friends. In meetings, 80% of decisions are made in 20% of the time, and 20% of your customers (products) account for 80% of your sales.
Of course, the Pareto principle doesn't hold true in every situation (mathematicians prefer to talk about a "64-4" rule, since 80% of 80 is 64 and 20% of 20 is 4). But anyone who wants to optimize their time must know that 20% of the time invested leads to 80% of the results.