Clicking this icon will display pages that you have added to your bookmarks. This can optionally be displayed in the left menu by toggling settings in User Preferences.
To add new bookmarks, hover your mouse over the page's entry in the side menu. You will see the same icon. Clicking the icon will shade in the icon and add the page to your boomarks.
You can click the lock 🔓 icon to prevent accidentally moving or deleting a bookmark from your list.
Feature Requests / Ideas
We value your feedback and ideas. Please raise any feature requests on GitHub.