What sort of gesticulation does Fortunado make and why doesn't Montressor understand in The Cask of Amontillado?

Fortunado makes a gesture known as "the sign of the mason," showing crossed arms and tapping the shoulder with a trowel. Montresor doesn't understand because he is not a Freemason and does not recognize the significance of the gesture, indicating that he is not part of the secret society to which Fortunado belongs.