Roughly 20% of inputs produce 80% of outputs.
(or 20% of causes creates 80% of consequences)
This applies to many events observed in the world of economy, biology, computing, sport and many other areas.
It means you often can identify 20% of work that will bring you 80% of outcomes (often to get to from 80% to 100% of outcomes requires 80% of effort)
Example: Sometimes when my kitchen becomes a mess and I don’t have 20 min for a comprehensive clean-up I am doing a 2 min cleanup. It is incredible how much one can achieve in two minutes and later on the task of finishing the cleanup doesn’t seem that big.
Other names: Pareto principle (see Pareto Principle on Wikipedia)