In The Da Vinci Code, what is a cryptex?

The cryptex in The Da Vinci Code consists of a matching mechanism contained within a cylinder whose purpose is to hide a secret message written on a piece of papyrus protected within the cylinder. This cylinder has its flat sides blocked so that nobody can see what is inside of it. In order to access the coveted information on the papyrus, one must have the "password" formed using the doughnut-shaped marble blocks that are connected by a brass rod and attached to the cylinder.  The marble blocks have the alphabet carved in each of its sides and you have to know "the code" and how to unlock the marble blocks until you make the correct word. It is, as described in the book, "like a bicycle's combination lock".

In the novel, the problem with the cryptex was that if the code is entered incorrectly, or if you attempt to misuse the cryptex, a vinegar solution within it will dissolve the papyrus containing the secret information.

Now, the word itself was coined by Dan Brown, and is a combination, as explained in the novel, of processes meant to covert essential information.

"the science of cryptology to protect information written on the contained scroll or codex".