Flask is a micro web framework written in Python that allows developers to create websites and APIs. It has a simple core but is highly extensible, only importing what is needed. Flask is easy to learn, follows Python conventions, and has excellent documentation. While small, it can handle large applications and is testable without making decisions like what database to use. Under the hood, Flask has only around 450 lines of code but 5000 lines of documentation and 1000 lines of tests, making it lightweight and efficient.