The Master page provides a simple way to lay out standard elements that should appear on every page. For example page numbers or copyright information.
The selected element is removed from the current page and added to the master page. It's appearance will not change but it will no longer be selectable and will appear on all your pages.
The user interface switches to editing the master page. You can confirm this in the Pages and Styles browser, the single page there now has the name "Master" and the Edit Master Page menu item now has a check against it.
The master page elements will disappear from the current page.