You can leave the message of the article blank, but if you do this the 'Upload Files' section won't be optional. It's recommended that you use the large text box in the first step of submitting an article to describe your files and when they were created.

What's the difference between the 'Author' and 'Your Name' fields?

That's for the people who want to add material that they didn't author themselves. The 'Your Name' field can be left blank if you want to post anonymously, but there has to be an 'Author' name.

What are keywords used for?

Tagging your article with keywords will help with making your article searchable, and provides an idea of what the article or files are about when listed in a category or on the main page.