what can I do to complete my missing information in Flask?

Learning Flask can be overwhelming. There are many pieces that need to fit together to make the whole. The difficulty in teaching Flask is trying to demonstrating functionality of the whole before all of the pieces are understood. Following the Flask lesson may be as complicated as all the previous Python lessons combined due to all the news pieces.

At the beginning, many of these pieces will need to be simply accepted as a "thing that works" with the understanding that theses pieces will make more sense as the course goes on.

So while there are other websites and lessons on learning Flask, it's a bit like asking to learn Flask before you learn Flask.

I suggest continuing on with the Treehouse course, while making a list of the unclear concepts that need more explanation. If the unclear concept doesn't become clear later on or at the end of the course, you'll have a great list of topics to research online or to ask about here in the forum. If you feel a specific concept needs more explaining immediately then a quick web search works well.

The main Flask documentation site is http://flask.pocoo.org/. A quick way to find specific help is to use the Google site: prefix. For example, to search for url_for I would use:

site:flask.pocoo.org url_for

Which leads to the link http://flask.pocoo.org/docs/0.12/api/

Where it says:

flask.url_for(endpoint, **values) Generates a URL to the given endpoint with the method provided

Of course, these leads to the question: what's an "endpoint". It's also explained later.

Note that site: can be used to search on Treehouse as well!

site:teamtreehouse.com url_for

If this isn't enough there is also the Flask tutorial by Miguel Grinberg, who has written many excellent books on Flask:

https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world

And definitely post back if you need more detailed help on anything. Good luck!!!