Langdon's having issues with his claustrophobia, seeing as he's locked in the cargo hold of a truck.
He and Sophie start to inspect the rosewood box.
Inside they find a polished white marble cylinder. Apparently, it's a cryptex, a device that Sauniére used to craft as a hobby. (Man, people in Paris have interesting hobbies.)
Made using blueprints found in Da Vinci's journals, cryptexes were a rudimentary way to send information over long distances in a secret, secure manner. (Back in his day, you had to rely on messengers—who weren't always trustworthy.)
So, they're built like bike locks: line up the right letters to form the password, and it'll open. Then you'll be able to pull out the scroll of papyrus (this is important) stored inside.
But. If you try to force it open or break it, the glass cylinder containing vinegar, around which the papyrus is rolled, will break—thus dissolving the papyrus and destroying whatever was written on it.
Sophie knows all of this because her grandfather used to make smaller cryptexes (cryptexi? cryptexus? what is the plural of cryptex?) for her birthdays, so she would have to solve clues and elaborate treasure hunts to get her gift.
(What a strange childhood.)
Since they don't know the password yet, Sophie and Langdon start puzzling over the symbolic meaning of the rose. (See our Symbols section for a full synopsis.)
Then a lightbulb goes off for Langdon. Something about sub rosa.
We'll find out what he's thinking about in another chapter, though.