You can save any score to your list of favorites.
The Favorite button with a heart on it is under the 'Download' and 'Print' buttons on the right of a score page. Clicking that button does three things:
It increases the number of favorites the score has, which appears next to a tiny heart above the buttons, by 1.
It adds you to the list of people who have favorited the score, which can be viewed by clicking on the number next to the tiny heart above the buttons.
It adds the score to your own list of favorite scores.
In the mobile app, this button is located in the three-dotted menu (the ⋮ icon) on the upper right of the score page.
Anybody can see the list of your favorites by going to your profile and clicking the 'Favorites' tab.
In order to unfavorite a favorited score, you need to go to the score page and click the unfavorite button (located in the same place as the favorite button).
Keywords: favourites.