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.
When you select the “Boxed Layout”; the individual containers and widgets appear inboxes.
You can find the setting under Appearance > Customize > Global > Container
You can change the background color from Customizer > Global > Colors > Base Colors > Base Colors.
You can apply boxed containers for selected pages or posts from meta settings.
The width of the container depends on the Site Layout settings.