A large part of communication nowadays takes place online. In order to provide users with more possibilities for expressing their individualities, Facebook brings Avatars to the United States. It was previously introduced for users in Australia and Europe, now if you are from the US you can use them as well.
This is how they look:
Basically, they are personalized stickers with an online avatar that every user can create based on their identity and preferences. Then they can be used in ways on the platform including comments and Stories.