Flask is a Python framework for web development.
When creating simple static sites, there is no need to use backend frameworks.
However, when creating complex and large sites that are backend heavy,
you will want to use a backend framework. Currently, the 2 Python frameworks
that are the most useful to learn are Flask and Django.
Flask:
Micro and light-weight framework that is easy to learn.
Django:
Complex, full-stack framework that is extremely powerful.
For smaller projects, Flask will work. However, for large ones, you might
consider learning Django. I suggest you learn Flask first and then Django.
Additionally, considering learning Node JS because even though it is in
Javascript, it is extremely popular.