The navigation pane is a tour guide for your document. It will help you quickly go to the section you want to check. To show the navigation pane, you need to check and format the headings first.
If you did not format the headings in your document, please check the article on how to create a table of contents to format the headings properly.
If you already formatted the headings, please see below how to bring up the navigation pane.
Step 1: Click the "View" tab from the ribbon;
Step 2: Check "Navigation Pane" in the "Show" group;
Step 3: The navigation pane will display on the left side of your document.