A polaron modulator was a device capable of compensating for extreme graviton stress.
In 2377, Bosaal, captain of a vessel trapped in the Delta Quadrant anomaly known as "the Void", attacked a Kinjal frigate and stole a polaron modulator, presenting it to Kathryn Janeway, captain of the USS Voyager, for use in escaping from the anomaly. When Janeway realized that Bosaal had murdered the crew of the frigate in order to obtain the modulator, however, she ordered it disconnected and Bosaal ejected from Voyager.
This forced B'Elanna Torres, Voyager's chief engineer, and the crew of a Nygean vessel, to construct their own polaron modulator. Eventually, they were successful, and Voyager and a number of other vessels were able to escape the Void and return to normal space. (VOY: "The Void")