As explained in the Container Overview article, typically a website can have two containers: 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 is the recommended layout for page builders like Visual Composer; where page builder requires themes to control the width of the container.