Flask is a web framework written in Python.
This means flask provides you with tools, libraries and technologies that allow you to build a web application.
Here a a few resources for learning Flask (I have not personally used them before):
https://youtube.com/playlist?list=PLzMcBGfZo4-n4vJJybUVV3Un_NFS5EOgX
https://realpython.com/tutorials/flask/
https://opensource.com/article/18/4/flask