The Manage Files tool in D2L is the file storage space for your D2L course. Every course site in D2L has its own Manage Files area, and in order to move files from one course to another, you need to Copy Course Components. Every file you upload into D2L is stored in Manage Files, and you can organize your Manage Files area much like you would organize your folders on your computer (note that students do not have access to Manage Files).
Here are some basic tutorials on Manage Files:
The Basics of Manage Files – What is Manage Files, and What is it Used For?
Linking Files from Manage Files to Other Tools in D2L