Hi, I had a problem with my all done cockboard, them I needed to import another one, it was my fault, but I can’t delete the previous one. It might be a bug.
On the question: do not delete the documents on your corkboard. That will delete your files completely.
The corkboard updates when you select a new subset of documents. I would recommend you start by going through the Interactive Tutorial that you can find in the Help menu. It helps a lot.
The corkboard shows all the documents in your project as index cards. Delete one, and the corresponding document will be deleted. If you need more room, try paging down? If the line of cards at the bottom is full, click on the last one, and hit return. A new card will show on a new line. The corkboard can hold millions of cards; don’t worry about running out of room.