When a certain segment on which you are reporting is important, you might want to know more about it and how it relates to other reports. This is often called breaking down a report by another report. A breakdown is the action of integrating two or more Analytics variables.
An example of a breakdown report is a breakdown of a custom traffic report called Age Group, in which you have gathered site visitors’ ages from a survey. This report allows you to see which age group is responsible for the most traffic on your site. However, if you have gone to the trouble of capturing something like the age of a visitor, you might want to know about more than just page views. You can generate a breakdown report about which search engine they use, what keywords they use, or where they are located geographically. You are correlating the age group and the search engine.
By default, there are no breakdowns when you first implement Analytics.