What's a keystone? Let's allow Dan Brown to educate you—a keystone is:
"an engraved tablet that revealed the final resting place of the brotherhood's greatest secret… information so powerful that its protection was the reason for the brotherhood's very existence." (2.20)
So it's like a treasure map for Grail hunters. (Woohoo!)
Symbolically, it's the final piece of the puzzle that a group of people have been seeking for centuries. The thing that's made it so tough to find is that people have taken the word "keystone" too literally.
As Langdon explains,
"Clef de voûte is a common architectural term. Voûte refers not to a bank vault, but to a vault in an archway. Like a vaulted ceiling."
"But vaulted ceilings don't have keys."
"Actually they do. Every stone archway requires a central, wedge-shaped stone at the top which locks the pieces together and carries all the weight. This stone is, in an architectural sense, the key to the vault. In English we call it a keystone." (48.8-10)
So people like Teabing, who have spent their lives seeking the keystone in order to find the Grail have been looking in churches and other places for literal keystones.
But as Langdon and Sophie discover, the keystone is more of a metaphor than previously thought (like everything else in Da Vinci Code), and is in actuality the cryptex, a device cleverly crafted by Saunière.