I noticed a heart icon on photos in the gallery. Is there a favorites feature?

Yes — guests can tap the heart on photos they love, and those liked photos get collected automatically into a special "❤️ Favorites" folder inside the album's file manager. It's a lovely little signal for you as the photographer, honestly: the favorites folder basically tells you which shots the guests themselves loved, which is free market research for your next album's cover or your portfolio. And guests like having a quick way to mark the ones they definitely want to download.