On Snapchat, the yellow heart emoji ? next to a friend's name indicates that you and that friend are best friends or "Super BFFs." This means that you both send the most number of snaps to each other compared to anyone else on your friend list. The more snaps you exchange with a friend, the longer the yellow heart will stay on your friend list.
It's important to note that the yellow heart is different from other heart emojis on Snapchat. For example, a red heart ❤️ next to a friend's name indicates that you've been each other's best friend on Snapchat for two weeks straight. There are also other colored heart emojis such as a pink heart ?, which indicates that you've been each other's best friend for two months straight.