"Favoriting" a photo is useful if you need to easily locate and access it from your library.
To favorite a photo, go to the Photos tab on the left-hand side, and scroll your mouse over the photo that you’d like to favorite. Click the heart outline in the left-hand corner of the image. The heart will turn solid white, which means it has been favorited. This item has been added to the favorite tab on the left-hand toolbar. It will be in the photos section.
To remove a favorite, simply click on the heart again, and it will be removed from the favorites tab.