Ablative armor is a protective material that functions by expending the energy of destructive forces on degrading the armor itself instead of whatever the armor protects.
A good example is the heat-shield used to protect space capsules and missiles returning to Earth from outside the atmosphere. An ablative coating on the re-entry vehicle vaporizes at high temperatures, expending the heat from friction with the atmosphere to keep the vehicle itself from growing too hot and melting.
Ablative Armor in Star Trek
Ablative armor is seen on newer Federation starships such as the Defiant-class, Prometheus-class, and Sovereign-class. It can protect a ship from multiple hits from Dominion weapons or Klingon disruptors before failing. Ablative armor allows a ship to stay in combat even after its shields have failed or briefly lower its shields to use transporters during combat.