On his blog he published an introductory post.
Just imagine having a medium sized application where everything is in the `app/ù folder, worse, every model is in the root of the app folder! At some point you will spend a lot of time looking for things because everything is bunched together.
This is what being modular is trying to resolve. You split of the business logic into different parts, which belongs together. If you're into Domain Driven Design, you can consider a module an aggregate.
Every module has its own routes/controllers/models/views/business logic/etc. Meaning every module contains a group of classes that all are related to each other in some way.