You can add text, watermarks, logos, or other images that appear in the same place in the background of every page of your document. These repeated elements are called master page elements. The more transparent you make the object, the more it fades into the background behind the primary content.
Page numbers are a great thing to place on the master page to ensure they are consistently presented.
The element will move behind any elements in front of it on the regular page.
When you're done choose Arrange > Edit Master Page again to return to normal pages (the checkmark next to the menu item will clear).