Space Wolves warriors are strong believers in charms and other blessed items, which they believe have the power to protect them in battle. A model protected by a runic charm may re-roll one failed armour saving throw per battle.
Errata — Chapter Approved: Codex: Space Wolves clarifications (v1.0): If a Runic Charm is combined with Terminator armour, it does not allow a re-roll of the Crux Terminatus invulnerable save that comes with the armour.