An example of a framework is Laravel, which is written in the PHP programming language. It provides a whole bunch of utility features out the box, from security, to database access management, to common front end user interface elements. When a developer is asked by a client to make a custom CMS, ERP or any other kind of back end system, they will typically use a framework like Laravel as a starting point.
Frameworks are pretty much always a good idea. They drastically speed up software builds (and hence reduce costs), they are typically open source (i.e. free), and the more popular ones are built by some of the best coders in the industry - meaning your project’s starting point is by default more secure and better built than if you went it alone.
Popular back end frameworks include Django, Laravel, Yii2 and Ruby on Rails. Popular front end frameworks include React, VueJS, Angular. Bootstrap is also a front end framework, although it focuses more on style and layout than interactivity.