A master template is the template that will apply to all the pages of a specific website.
To edit a master template:
1. Click the Pages tab on the upper toolbar menu
2. Click the Edit button in the Master Template area of the menu
3. You are now in the editing mode for the Master Template, all changes made here will apply to the master template
4. When done editing click finish to return to the editor
Additionally changes made to the page itself in the editor will by default apply to the master template.
To learn how to disable this option: How to edit a page's design without affecting the master template?