A trowel is a tool used to spread the cement or mortar that holds bricks or stones together. For that reason, it has become a sign of the Freemasons (or Masons), a secret fraternal organization. Within the Masons, the trowel symbolizes the "cement" of brotherly love and affection that ideally holds the members together without enmity or dissension. 

When Montresor fails to comprehend a sign Fortunato makes that shows him to be a mason, Fortunato says, "You are not of the masons."

Montresor repsonds

"Yes, yes," I said; "yes, yes."

Fortunato is contemptuous, giving us an insight into why Montresor might feel injured by him. Fortunato says:

"You? Impossible! A mason?"

Fortunato then demands a "sign" and Montresor replies:

"It is this," I answered, producing from beneath the folds of my roquelaire a trowel.

Again, Fortunato shows his disbelief and disdain:

"You jest," he exclaimed, recoiling a few paces. 

Of course, this exchange about being a mason is ironic. Montresor is not a mason in the sense Fortunato means, but another kind of mason. The word mason is thus a double entendre. To Fortunato it means a member of a secret society, while to Montresor it means a person who builds a wall. The trowel Montresor shows also has two meanings: the brotherly love and unity it symbolizes within the Masons and the hate it symbolizes to Montresor.  Ironically, Montresor is a "mason" who offers the opposite of brotherly affection, and his trowel delivers the opposite of the brotherly love when Montresor uses it to wall up his enemy so that he dies a horrible death. Rather than a symbol of unity, it becomes a symbol of separation.