What is the TL;DR of the category? What do tools in this category let you do?
The application framework is the heart of a dynamic web application. A web server takes in the request, but these frameworks handle and manipulate all the data tied to that request based on the behavior of the application before sending it back.
Why would I need to use it? Why would I need to start using it?
You would need to use this when your website grows to something more than just static HTML files. Growth spurts include handling users, dynamic content, and connecting to a database.
Where in the Back-End design/pipeline is this category of tools used? Learn more.
These reside on the back-end past the web server. They are usually the last stop for an incoming web request before being sent back out. An intermediate interface is needed between the framework and the web sever so they can talk to each other.