As explained in the Container Overview article, typically a website can have two containers viz: Primary Container and Secondary Container.
Primary Container is where the main content appears, whereas Secondary Container consists of the sidebar and various different widgets in the sidebar.
“Full Width / Contained” layout merges the primary and secondary containers and displays in a single container in a clean format.This container will display in a container width, leaving a space around the container.
You can find the setting under Appearance > Customize > Layout > Container
Full Width / Contained layout is great for websites that need a clean design without any boxes. We set this layout as default in our integrations with some plugins such as WooCommerce etc.
Full Width / Contained is also the recommended layout for page builders like Visual Composer; where page builder requires themes to control the width of the container.
You can apply boxed container for select pages or posts from meta settings.
The width of the container depends on the Site Layout settings.