Magma Armor is a Pokémon ability introduced in the third generation. It prevents freezing
Pokémon with Magma Armor cannot be frozen while the ability is active. If the ability is just gained or was ignored, the Pokémon will thaw their next turn. If it was frozen, it consumes the Aspear, Lum or Pumkin Berry it was holding before the ability activates. If Limber is no longer the Pokémon's active ability, it does not thaw until it has switched back in.
If a Pokémon with Magma Armor is in the party, it reduces the number of cycles to hatch Eggs in the party by half. It shares this effect with Flame Body though, it does not stack with other Pokémon with this ability or Flame Body.