From UFStarfleet Wiki
Ablative armor was development during the Borg defensive weapons project along with pulse phaser cannons and quantum torpedoes. Ablative armour provided an extra layer of defence for the vessel against enemy fire if the shields failed. It worked by forming a protective layer over the hull surface capable of rapidly dissipating the energy impacts from directed energy weapon fire. It is standard fitting on Promertheus class starships and optionally added to other combat orientated starships such as the defiant class.
It is of note that under certain conditions, chroniton particles generated by Romulan cloaking devices can become lodged in the ablative armor matrix. On at least one occasion, a temporal surge caused by the explosion of a microscopic singularity shifted the chroniton particles in the hull into a high state of temporal polarization, thus causing a transporter beam to be redirected through time
The Borg's long-range tactical vessels were also equipped with ablative hull armor. Hirogen outposts, designed as training facilities by means of holodeck technology, utilized three layers of ablative armor to protect the grid's holoemitters.