The Pareto principle, also known as the 80-20 principle, refers to a statistical distribution common in the work-outcome dichotomy.
Perhaps the most common statement is: 80% of results are produced by only 20% of the effort. But this law applies to many situations.
How can this help us?
Knowing this almost universal distribution allows us to determine the most important actions and to maximize our productivity. If 20% of the actions I can perform lead to 80% of my results, it seems wise to be able to identify them, and then delegate certain tasks to improve my productivity. This comes back to the idea of specialization, and the idea of not falling into the "Jack of all trades, master of none".