Masters are useful when you want the exact same content (e.g. a logo) to be replicated on a bunch of different screens. Masters make this easy, and if you decide to change your logo, you only have to change it in one place and those changes will be automatically be reflected on all the screens.

To create a master in Editor, click on the plus icon underneath the Edit tab.

You’ll see a dialog that lets you pick a pre-built master from your libraries. Most good libraries should also include some blank masters as well.

Click on a master to add it to your project, and then click on 'New Master'. 

Once the master is added to your project.  You can edit it to your liking.  To edit a master in your project:

Open the master by double clicking on it in the Master panel or the workspace. That will open the master for editing.

Add or edit widgets in the master. Any changes you make will be immediately reflected on any screen where that master is used.

 To add a master to a page:

Open the Masters panel on the left.

Drag a master from the panel on to the page.

You should be all set! If you have any questions, feel free to contact Support for more help.