A Rune Priest's staff is carved with many mighty runes of nullification which protect him against enemy psychic powers. In game terms it has the same effect as a psychic hood (see Codex: Space Marines).
Errata — Chapter Approved: Codex: Space Wolves clarifications (v1.0): A Runic Staff counts as a single-handed weapon AND a psychic hood, and should therefore cost 25 points.