Perhaps the best explanation I've seen [medium.com] that explains what Snapchat is:
“Jumping into their experience,” I think is probably the closest thing I’ve heard to a unified theory of what Snapchat is. It connotes an active give and take between friends (and more recently, influencers). It foreshadows the importance of the doodles, stickers and filters that have come to define much of Snapchat, which are more about giving us an excuse to share anything — profound or mundane — than posing for an eternal self portrait. It’s something that only really works when the capture and consumption device are the same, and where the output — vertical photos/videos — fully immerses you in each experience shared with you.