Montresor does not make his plans explicit because he pretends to be sharing rare wine when really he is planning murder.
The word “explicit” is used in the beginning of the story. Montresor describes how he made sure that none of his servants knew what he was up to.
I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned.
Indeed, Montresor does not make his plans explicit to Fortunado because then Fortunado would know that he plans to kill him. Instead, Montresor wants to commit the perfect murder because Fortunado has committed some injustice against him. So Montresor tricks him, telling him he wants to show him some rare wine and that’s why he is taking him into the catacombs.
If Montresor was explicit about his plans, he would have said something along the lines of, "Hey Fortunado, can you come with me so I can kill you to avenge this terrible injustice you did me?" Chances are he would not go.