What does "dirty" mean?

This question came from Slack. An user asked there which situation could result in a "dirty build directory" and why it should be removed.

We don't have a formal explanation about .dirty on Docs, because it's more like an internal stuff to avoid poisoned cache. But since the user is able to read an warning about "your build folder is dirty and will be removed", I think we could add some section explaining why we considered that folder as "dirty".