The tripods refer to the only source of light inside Prince Prospero's party. None of his room have lamps or candles or lighting of any kind. The tripods are like large standing fires and they stand behind the windows of each room.
"But in the corridors that followed the suite, there stood, opposite to each window, a heavy tripod, bearing a brazier of fire that protected its rays through the tinted glass and so glaringly illumined the room."
The tripods, because they are fire, create images on the walls of the room and the images are particularly grotesque in the Black room or the final room. The images that are produced by the tripod in the Black room are what keep people away from that room, just like faded youth, old age, and especially the Red Death can be so grotesque that people will stay away, the tripod keeps people away. The final room's light was said to be, "ghastly in the extreme, and produced so wild a look upon the countenances of those who entered, that there were few of the company bold enough to set foot within its precincts at all."