Flask is a web application light-weighted or micro framework written in Python. It provides tools, libraries, and mechanics that allow the developer to develop a web application without writing low-level code like thread management and protocols.
Python Flask can be used for building blog websites, commercial websites, Hackers news clones, Normal content sites, etc. but it will not enforce any dependencies or tell you how the project should look like.
Also, watch this video to create a simple application in Python Flask: