Magma Armor is an ability introduced in Generation III. So far, 1 Pokémon has this ability.
Magma Armor prevents the Pokémon from being frozen. Gaining Magma Armor (such as with Skill Swap) will thaw the Pokémon as soon as it enters battle. If the Pokémon is holding an Aspear or Lum Berry, it will be eaten before Magma Armor thaws out the Pokémon.
Outside of battle
If there is a Pokémon with Magma Armor in the party (even if fainted), the number of steps needed to hatch eggs in the party becomes halved. This same effect can be reached using Flame Body, but cannot stack.
Pokémon with Magma Armor