An "amontillado" is a type of wine produced in Spain. The first thing that is significant about this is that the story is set in Italy and thus this wine would have been imported, making it substantially more expensive than readily available Italian wines.
Next, amontillado is a type of "sherry." This term is an anglicized version of "Jerez," the Spanish town from which this type of wine originates. Sherries are fortified wines, meaning that extra alcohol is added to the barrels as they age, making their alcohol content approximately 50 percent higher than ordinary wines, with the darker, sweeter sherries having slightly higher alcohol contents than the paler, drier ones.
Sherries range from very dry and pale in color (types termed "fino" and "Manzanilla") to sweeter and darker varieties (including "Oloroso" and "cream"). An amontillado, named after the Montilla region of Spain, is in the middle of the sweetness and color range for sherries, having a medium amber color and a distinctively nutty flavor. It is usually consumed in small quantities as an apperitif rather than accompanying a meal.
Amontillado is a type of wine, specifically, a dry, red sherry, named after the Montilla region of Spain. The reason this is significant, is that a cask of this type of wine is very tempting to someone who is quite the fan of alcohol, as the one who had wronged the narrator was in this story. It is also very expensive, so to find a large amount of it hidden away in a wine cellar is quite the fortunate event. What person wouldn't want to come into possession of something worth possibly hundreds of dollars? It definitely proved too tempting to the one who wronged the narrator in this story, as it leads to his demise in the end.