Ablative armour is an additional reactive plating that imposes a counterforce to incoming weapon fire. The armour works by first dispersing incoming beam energy across the hull of the ship where, after reaching an undisclosed threshold, the energy causes part of the armour to boil away, taking with it a large fraction of that energy. The effect also creates a modest vapour cloud, which effectively disperses the incoming beam further, causing it to do less direct damage to the hull.
The ablative armour matrix was first deployed on the USS Defiant and later on the USS Prometheus. The Defiant was able to withstand several Jem'Hadar weapon blasts before the ship started taking damage.
It should be noted, however, that the armour is not a hull replacement, but a supplement, and must be replaced over time due to the boiling away process.