Langdon smartly pulls over so Sophie can take the wheel.
While she drives, he gets a bit manic about what the key could mean. Maybe he'll find the Holy Grail…
He knows two things: Leonardo Da Vinci knew where the Grail was when he was alive, and that the documents hadn't been moved since.
There's a bunch of conspiracy theories about Da Vinci's works and the possibility of the Grail's location being hidden within, but Langdon doesn't really put a lot of stock in those.
Most recently, there was a scandal with Da Vinci's Adoration of the Magi, which was revealed to be painted by an imposter who filled in an underdrawing incorrectly—possibly subverting Da Vinci's true intentions for the piece.
Sophie wonders if the key's to the Grail's hiding place. Langdon scoffs at that possibility, because there's no way her grandfather could've been the Grand Master or one of his three sénéchaux.
Plus, even if he were, he wouldn't have entrusted her with the key.
They find 24 Rue Haxo, and realize the address leads them to an incredibly modern, sophisticated Swiss bank.