Master page contains shared elements that can be populated on more than one page.
To add a new master page choose Page > Insert Master
The master page will appear in the page list
To select master page for a page choose Page > Setup and select one of master pages in Master or Overlay fields.
Items from Master page will appear beneath page items
Items from Overlay page will appear over page items
Master Page contains shared items, for example this side-menu:
News Page, left padding is increased to move flowing elements right and to create blank space for master elements:
Combined Page visible in the browser contains both master and page items: