As Rails applications grow they see a lot of the same problems that any enterprise level software project will see at some point: longer running test suites, more complex code interaction, and a rising requirement to keep "all that stuff" in your head at once.
Rails may have been a framework for single applications in the past, but it nowadays has some features that allow you to tackle bigger projects with more ease. We'll cover Rails code structuring techniques like unbuilt gems and engines that you can use to get faster test suites, cleaner structures, and more flexible apps.