What is CodePen Projects?
If you're familiar with the term, you can think of it as an IDE (Integrated Development Environment) for building websites. Like all of CodePen, you use it right in your web browser.
You have a sidebar with all your files. You can create new files and folders there (even by drag and dropping from your computer), as well as delete, duplicate, and rename them.
You can view, edit, and rearrange your files in separate tabs.
Files can be preprocessed and tidied.
There is debugging in that your code can be checked for errors and you will be notified of problems.
You can preview what you are building as you build it.
... read the whole story at blog.codepen.io.