This Emacs minor-mode creates an automatically updated buffer called `*Ilist*` that is populated with the current buffer's imenu entries. Imenu-list is usually configured so that the imenu structure is displayed in an automatically resized vertical Emacs window (a sidebar).

An imenu tree can be used as a table of contents for text and markdown documents, or for adding modern IDE features to Emacs--features like an overview of function and variable definitions. Speedbar also provides this functionality; however, imenu-list doesn't require X11. Unlike speedbar, imenu-list shares the existing Emacs frame. X11 windows are called frames in Emacs, so another way of saying this is imenu-list doesn't require a second X11 window with its own window decorations.