The atlas shows your progress on the different paths of the game. Bookmarks are used to change the path, and arrows on the page are used to browse the path's pages.

There are three different paths to explore. First one is the tutorial path that you complete before you are able to enter the story path. You can return to tutorial levels and restudy the concepts again if you forgot something. Story path introduces new enemies and provides the best rewards (gold chests). Replaying previous levels gives you rewards too: silver chests for the current and previous page, bronze chests for earlier pages. Nightmare path makes the whole experience more challenging and gives additional rewards.

TIP: Tapping a bookmark of a path you're already on takes you to the end of your progression on that path.