The Ablative generators were an advanced armor technology used by Starfleet in the 25th century. Shield emitters generated a nearly-impenetrable layer of armor around the hull of a ship so equipped. This defensive technology was acquired by the crew of USS Voyager in 2377 from Admiral Kathryn Janeway of an alternate timeline. It could barely be penetrated by disruptors or stop the ship with tractor beams.
In the alternate timeline, ablative generators were developed as an anti-Borg technology by Captain Janeway. Invented some time after they encountered the transwarp hub hidden inside a nebula in 2377, but chose not to investigate it further. She used the technology to protect Voyager against the Borg on their voyage back to Earth. By 2404, ablative generators had not yet become standard equipment on Starfleet vessels. Ships like the USS Rhode Island were not equipped with them. Also, the Klingons did not yet posses ablative generators. During that year, Admiral Janeway embarked on an illegal mission to change history by travelling back in time to help secure Voyager's safe return to the Alpha Quadrant seventeen years early. Janeway utilized a Starfleet Command shuttlecraft, the SC-4, equipped with ablative generators when meeting with Korath, a Klingon scientist, who had developed the chrono deflector, a device capable of generating temporal rifts for time travel. Janeway stole the chrono deflector and used the ablative armor to defend the shuttle against nadion discharges fired by Korath's Negh'Var-class warship in a retaliatory attack. After successfully travelling back in time to 2377, Admiral Janeway assisted Voyager's crew in manufacturing and installing the advanced armor technology aboard the ship. The crew used the armor to defend themselves against Borg attacks while attempting to access the transwarp hub located in grid 986. When the Borg Queen assimilated Admiral Janeway, she also assimilated knowledge of how to construct the armor technology, but was killed by the neurolytic pathogen Janeway was infected with. Eventually Voyager's armor integrity was depleted down to to six percent by a pursuing Borg sphere and the crew had to allow the ship to be captured. They were, however, able to use transphasic torpedoes to destroy the sphere upon exiting the transwarp conduit near Earth. (VOY episode: "Endgame")