if you want to hide something from Navigation portlets and/or the menus at the top of the page, 
  1. go to the content item and click Edit.
  2. click on the Settings tab.
  3. check Exclude from navigation.
  4. click Save
  5. Because the navigation menu will show the current item, you will have to go to another page to verify that the the item no longer appears in the navigation menu.

Items hidden in this manner will not show up in either Navigation portlets (vertical menus that appear on either side of your page) or the top navigation bar (which is part of the header that appears on every page). However, whenever you are viewing a content item, that item will appear in in the navigation menus until you move to another page.

It is not possible to exclude a content item or folder from the top navigation menu without hiding it from Navigation portlets, or vice versa.