Off Canvas layout is a common UI pattern for modern applications, especially on mobile devices. The concept is aimed at saving space on your page, allowing you to pop out a “drawer" of additional information. This helps reduce clutter on the main page but still provide access to important data when needed without leaving the page context. You can see an example of the runtime behavior at the top of this post.
Oracle JET provides this type of "off-canvas" behavior as a built in component, and they have a demo of it working as part of the cookbook here.