A website generally has the main wrapper followed by the container.
- The main wrapper consists of header, content, sidebar, and footer — the entire website;
- And the container is the child wrapper that includes content & sidebar.
When you select Full Width Site Layout, the main wrapper of the website goes edge to edge of the browser. Whereas the content is managed by the container width.
You can find the setting to manage Full Width Site Layouts under Appearance > Customize > Layout > Container > Site Layout.