A Master template is the outermost template that houses your Content template. They're commonly used for the websites header, footer and navigation systems. Acquia Cohesion allows you to layout your Master template using the Layout builder. You have the option to create multiple Master templates if you want different areas of your website to have different outermost structure.
Master templates are optional. If you are using Acquia Cohesion on an existing website with its own theme, you may not want to use a Acquia Cohesion master template and instead rely on your themes template.
For more information see: