This is a folder to contain any extra exercises, such as the "Learn By Doing" blocks in the lessons.
It's an area for you to create experiments, take notes, spin up new sandboxes, and generally put anything that isn't a lab or part of your module project.
Remember that for your module project, you want to create a new standalone GitHub repository for it. You can then link to it from your main GitHub profile page, and it'll be a great portfolio piece to show off to potential employers.
When it comes to your labs, you can mostly do them in the labs folders. Unless the labs tell you otherwise.
Note: In order for GitHub to display a folder, a file must be present within it. Therefore, this readme file has been replicated in the sub-folders located here.