A backend framework is a set of ready-made tools and patterns that helps developers build the behind-the-scenes part of an app. It is like a prepared workshop with common tools already arranged.
For example, a framework may provide ways to handle web addresses, receive form information, and connect to stored data. Developers add the rules specific to their own service instead of starting every common task from scratch.
A framework does not decide what an app should do, and it does not remove the need to check security and behavior. It provides structure that developers still need to use carefully.