To understand what Flask is you have to understand a few general terms.
WSGI Web Server Gateway Interface (WSGI) has been adopted as a standard for Python web application development. WSGI is a specification for a universal interface between the web server and the web applications.
Werkzeug It is a WSGI toolkit, which implements requests, response objects, and other utility functions. This enables building a web framework on top of it. The Flask framework uses Werkzeug as one of its bases.
jinja2 jinja2 is a popular templating engine for Python. A web templating system combines a template with a certain data source to render dynamic web pages.
Flask is a web application framework written in Python. Flask is based on the Werkzeug WSGI toolkit and Jinja2 template engine. Both are Pocco projects.