The terms “masons” and “brotherhood” are examples of sophisticated social groups, and Montresor pretends to belong to one while Fortunado does.
Montresor in convinced that Fortunado did him some kind of injury, in the form of an insult he does not describe. He considers himself an equal to Fortunado, even though he does not seem to be.
I broke and reached him a flagon of De Grâve. He emptied it at a breath. His eyes flashed with a fierce light. He laughed and threw the bottle upwards with a gesticulation I did not understand. (p. 6)
Fortunado is aware that Montresor is not really a mason, and not one of the brotherhood. He is a pretender, thinking that he is someone he is not.
Of course, the word “mason” and “brotherhood” are also ironic because they are terms of friendship, and Montresor is about to kill Fortunado—by building a brick wall in front of him.