What are layouts?
Layouts are responsible for determining which placeholders will appear on the pages of your web site. You find them organized by folders in the Sites and Channels directory of the CMS
This is done first with the placeholder settings, that will tell you which type is used and what file will fill it. Then, in order for the layout to become functional, it must be linked to a template. Thus, the page code where the placeholder will be displayed will know exactly where the information comes from.
Important
In order to work, each layout must be linked to a single template.