Rails is a framework
Ruby on Rails is a framework. A framework means there is strong philosophy behind the design of Rails. It provides all the essential components that we need to build web applications.
p.s. If you’re confuse a framework with a library, you can find their difference here: library vs framework.
For example, Rails provides a Model-View-Controller architecture, Assets Pipeline, I18N, Test-Driven Development.
Here is the Model-View-Controller architecture that Rails follows:
Rails also has 2 main principles: Convention over Configuration and Don't Repeat Yourself.