Best explanation of the Pareto Principle (80/20 Rule) I’ve come across:

The point of the Pareto principle is to recognize that most things in life are not distributed evenly. Make decisions on allocating time, resources and effort based on this:

• Instead of 1 hour on a rough draft for an article you may write, spend 10 minutes on 6 outlines for a paper / blog article and pick the best topic.
• Instead of investing 3 hours on a website, spend 30 minutes and create 6 different template layouts.
• Rather than spending 3 hours to read 3 articles in detail which may not be relevant to you, spend 5 minutes glancing through 12 articles 1 hour and then spend an hour each on the two best ones 2 hours.

These techniques may or may not make sense – the point is to realize you have the option to focus on the important 20%.

Lastly, don’t think the Pareto Principle means only do 80% of the work needed. It may be true that 80% of a bridge is built in the first 20% of the time, but you still need the rest of the bridge in order for it to work. It may be true that 80% of the Mona Lisa was painted in the first 20% of the time, but it wouldn’t be the masterpiece it is without all the details. The Pareto Principle is an observation, not a law of nature

via Understanding the Pareto Principle The 80/20 Rule | BetterExplained.