The Pareto principle (also known as the "80–20 rule", and the "law of the vital few" and the "principle of factor sparsity") states that 80% of the effects comes from 20% of the causes. This is the basic rule of thumb in business (“80% of your sales comes from 20% of your clients”), but can also be applied to design and usability. For instance, dramatic improvements can often be achieved by identifying the 20% of users, customers, activities, products or processes that account for the 80% of contribution to profit and maximizing the attention applied to them.
The pareto principal stipulates that a high percentage of users will perform a low percentage of actions. Meaning that most of your users are going to go to a small percentage of pages. Or in terms of web applications that most of your users will perform a small percentage of tasks.
Using this principal we can identify what that small percentage of actions that most of the users are performing (using analytics, research, interviews, etc…) We can then put higher emphasis on those tasks and actions to make the site easier to use. Sometimes this can lead to the inclusion of a new navigation, or altering the homepage to make finding and accomplishing those tasks easier.