Yes, that would be the “frisket” I mentioned.
I think self adhesive mylar works, but you need to be careful to apply it without getting any bubbles.
You might be surprised at how well spray paint or wax/tar (also called “soft ground” to intaglio printers) works.
People have been using the process for a very long time.
Here are a few good links to learn more:
http://www.wretchedetcher.com/etching-tutorial/etching-apply-ground.html