Ablative armor is an additional reactive plating that imposes a counterforce to incoming weapon fire. The armor works by first dispersing incoming beam energy across the hull of the ship where, after reaching an undisclosed threshold, causes part of the armor to boil away, taking with it a large fraction of that energy. The effect also creates a modest vapor cloud, which effectively disperses the incoming beam further, causing it to do less direct damage to the hull.
The ablative armour was first deployed on the USS Defiant, and then 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 armor is not a hull replacement, but a supplement and must be replaced over time due to the boiling away process.