To use it, you just have to select the scenario you like most from the two big lists you'll find in the app. The first includes official maps and scenarios designed for Minecraft by professionals. The second includes unofficial maps developed by gamers who want to share their creations with the world. Both types come with descriptions of what you'll find in the settings so you'll know what you're downloading.
The maps include many challenges that you'll have to overcome. To access the download list just open Minecraft and click on any of the new options that appear at the start of the game.