The course index is the collapsible drawer on the left of the screen in Boost and Boost-based themes. It displays course sections and elements. As you move up and down the central course area, your location is highlighted in the Course index. Clicking on an item in the Course index will open it up directly in the course central area.
Activities and resources which are indented in the course page are also indented in the Course index.
Text and media areas also display in the Course index.