The Pareto Principle is also known as the 80/20 Rule, a principle which has applications in business, productivity and economics. It has been used to explain a number of phenomena and to help leaders prioritize their efforts.
Simply put, the Pareto Principle states that 80% of the outcome is dependent on 20% of the efforts.
In companies, 80% of the business comes from 20% of the clients, while 80% of the output often comes from 20% of the employees. Similar distributions are observed in math and natural systems.
In fitness, we can examine Pareto Efficiency and use it to our advantage.
With regards to weight loss, 80% of the results are determined by your efforts in the kitchen with regards to nutritional choices. Upon further examination, 80% of weight control can be attributed to controlling blood sugar levels through carbohydrate choices.
With weight training, the principle applies to what exercises you do. You should favour certain such as deadlifts, squats, presses and pullups to get both the physique you want and the balanced strength gains that these compound exercises elicit. There are hundreds of exercises to do, but understand that 20% of these exercises will give you a greater result than the rest of the exercises combined.
In endurance training, most coaches and athletes will understand that the hard efforts only come 20% of the time. The remaining 80% of the training should be spent on easy efforts. In this example, it is not only more efficient to stick with the 80/20 rule, but also necessary for preventing overuse injuries and overtraining.
Tim Ferriss made the term "Minimum Effective Dose" popular in his book "The 4 Hour Body".
The term effective dose is a pharmacological term referring to the mimimum amount of medication required for effectiveness. He's applied this principle to learning things quickly by identifying what the minimum dose or, in Pareto terms, where the 20% effort needs to be directed. Here's his Google talk which describes his philosophy.