It’s sorta like a collection book. It shows you every item in the game at each tier (epic/leg/myth etc). And you can unlock each new “level” of it by doing things such as pvp, getting said items etc.
It’s a WIP feature that is only seen on newer accounts. I don’t know for sure if they’ve rolled it out yet for everyone. But yeah, hopefully when they do, Sarah/Mohadib will fill us in on all the other details.