A spring-loaded cover protects your papers from the rain whilst you write. When you're not writing, the cover closes to keep your documents secure and dry (secured by a long strip of velcro.) Since the cover is see-through, you can still look at your documents when the cover is closed.
A clip inside the cover keeps the documents secure. There's also a clip on the back of the clipboard (two clips on the A3 model), so you can still use your Paperdry clipboard when it's not raining.