The nitre is significant because it foreshadows Montresor’s murder of Fortunado, because he comments on how Fortundado should not go into the vault because he has a cold.
Montresor has been planning Fortundao’s demise, but he does not want Fortunado to know it. Fortunado has insulted him, but he is likely not aware of it. He uses reverse psychology to get Fortunado to want to come by telling him she should not come.
“My friend, no… the severe cold with which I perceive you are afflicted. The vaults are insufferably damp. They are encrusted with nitre.” (enotes etext p. 5)
Nitre is white dust found on the walls inside caverns. When Fortunado coughs, they both say it is the nitre. Montresor tells Fortunado to go back, but he does not agree. Montresor says that it “hangs like moss upon the vaults” (p. 6). He is trying to keep Fortunado off guard. In the end, he successfully blocks Fortunado up in the wall, killing him.
This is not such an easy question, but there are at least a couple of plausible answers.
Nirtre, or potassium nitrate, is a mineral deposit sometimes left on damp walls underground, as in cellars, caves, or basements. As Montresor leads Fortunato deeper and deeper into the wine cellar/crypt, he calls his attention to it. It is extensive and is described by the narrator as "white web-work." It could symbolize the web in which Montresor has trapped his enemy, Fortunato, whose death he will soon bring about.
Nitre has been used as a preservative. Because Montresor plans to entomb Fortunato while he is still alive, it could represent Montresor's desire to preserve his enemy's corpse for all time as a macabre monument to his successful revenge scheme.