I am still confused about check-in and check-out. What do they do?

check-out: A way to "lock" a file while you are making changes so that nobody else can edit the details or upload a new version. It is also like checking out a book from the library. In order to edit a document you must check-out the document first.

check-in: Used when you have completed your modifications. It places the document back as available, or if using workflows, sends the new revision into the approval workflow process.