Primary menus are the main menus. The Main menu's links drive the main navigation structure for your site, and are often displayed prominently across the top or side of the site. The Secondary menu is often used for items like the copyright and privacy notices and are often displayed at the bottom or very top of the page in smaller text. You can specify which menus are used for the main and secondary links.
For more details refer - https://www.drupal.org/docs/7/working-with-menus/working-with-menus