The heart-shaped icon acts as a Favorites button. The color of the icon can indicate if a video has been or hasn't been added to Favorites.
Video is not added to Favorites.
Video has been added to Favorites.
The "Add to Favorites" button can be found in the following sections:
1. While browsing can be found in the "Quick Access Menu".
2. In the "Video Player Menu", in the top right corner, above the video player.
All videos added to Favorites can be found in Account > My Collections > Favorites.