I believe that the answer to this question is that Fortunato asks Montresor (more or less) if he is a Freemason.
The Freemasons were (and are) a secret society. Many of their members were very important in society. Because they are secret, they do not walk around with badges saying "I'm a Mason." Instead, they identify themselves to one another with secret signals. This is what Fortunato is doing when we are told
He laughed and threw the bottle upwards with a gesticulation I did not understand.
Montresor does not understand the gesture because he is not a Mason. Fortunato follows this gesture up by telling Montresor that he must not be a Mason since he did not understand the gesture.
Please follow the link for an article about Masonry.