A Layout is something like the outer frame of a website. It is the largest entity in our templating-layer and wraps all the other components of a website on every page.

It defines the very basic structure of a page, like the place of the header, the footer and the main navigation. There will probably not be many of them. Most websites will be fine with just one Layout.

Note: On the configuration level (project_theme/Configuration/TypoScript/Layouts), the template for each page is determined, based on its BackendLayout: