Ablative armor is a protective hull plating that absorbs and disperses weapons fire, sacrificing its own outer layers to keep a ship intact.
Instead of relying entirely on shields, ablative armor lets incoming energy burn away a sacrificial surface layer, spreading the damage across the hull. The USS Defiant was built with this kind of armor as part of its design as a dedicated warship against the Borg.
A far more advanced version appeared when an older Admiral Janeway travelled back in time to help Voyager get home. Her ablative armor generator could cover the ship in armor plating within seconds, making Voyager almost impervious to Borg weapons during its passage through a transwarp conduit.