Clicking on the heart to the left of a question will mark that question as a favorite.
To remove a question from the favorites, click on its heart again.
Questions marked as favorites can be easily found later by applying the Favorites filter.
Marking a question as a favorite can be especially useful for hard-to-find questions or any questions you use frequently.
Favorites are a per-user setting. Marking a question as a favorite will not affect other users.