A Document Library is a place to store documents. These documents can be viewed or downloaded by anyone viewing the site. It is useful for posting assignments/forms that others might need.
Below is an example of an empty document library.
To upload files, drag the file from your computer onto where it says "Drag files here to upload". Immediately the files are uploaded. If drag and drop does not work, you can click on the "Upload" button and select a file from your computer.