This tutorial actually starts with Miguel Grinberg's Flask Mega-Tutorial at http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world and you should complete Part I, II, III of that tutorial first. This begins at Part IV and deviates at that point.
This tutorial also assumes you have basic knowledge of MongoDB: Setup, Running, Querying. For tutorial on MongoDB admin: https://university.mongodb.com/courses/M102/about It also includes some general intro in queries and how MongoDB works.
MongoEngine resources: http://docs.mongoengine.org/ This is documentation about the mongoengine package used in this tutorial.
Now starting with Part IV: Flask MongoEngine Tutorial