What's so confusing? It's a social media network that's focused on sharing photos and videos. I don't use it (or Facebook or Snapchat or Twitter, for that matter), but I understand why other people find it appealing.
These kids today, with their Instantgraphs and their Twisters and their Snoopchits. When I was a kid, all we had were Polaroid snapshots AND WE LIKED IT!