Flask is a lightweight Python web application framework. It makes clever choices to be simple but powerful, such as using existing libraries like SQLAlchemy and Jinja, and keeping boilerplate down to nothing. This makes it very compelling when you want to create “just a few functions” sized applications.
But I’d strongly recommend skipping Flask. If you ever need to expand your application, Flask won’t have your back. Although Flask does have extensions, they are too immature, too few and not connected enough to serve you well. If you want to add users, a REST API, and admin interfaces or other features that are “table stakes” for a framework you will find yourself stranded, trying to tie together poorly maintained pieces and doing manual work.