Breakdowns let you break down Analytics variables into more specific data. For example, you can break down the visitor age group report/dimension by the type of search engine the visitors use.

Note: Breakdowns were previously divided into subrelations (conversion reports broken down by conversions report) and correlations (traffic reports broken down by traffic reports.) These limitations are no longer in place. You can break down conversion variables (eVars) by traffic variables (props) and vice versa.
Tool Link
Analysis Workspace Break down dimensions and dimension items
Ad Hoc Analysis Break Down Table Data
Marketing Reports & Analytics Breakdown