Component responsible for displaying the indicator, in form of an animated progress bar, when user is navigating to the other page. It gives the user impression that something is being loaded in the background and he should wait for the next page to be loaded.
Technical details:
It is as simple script that adds animated element to the page on beforeunload event, and removes it on pagehide event.
Related template: src/Magento_Theme/templates/redirect_progress_bar_script.phtml
Related configuration (on/off switcher):
src/etc/view.xml -> redirect_progress_bar