The sidebar remembers your current page, highlights the active item, stays in a fixed position on the page, and more. This page explains a bit about how the sidebar.
Edit me
Accordion sidebar feature
The sidebar allows you to view the table of contents and navigate through the different topics.
Fixed position sidebar
If the your view is tall enough, the sidebar remains fixed on the page. This allows you to scroll down the page and still keep the sidebar in view.
Collapsing the sidebar
You can collapse and hide the sidebar by clicking on the Nav switch in the topbar menu