When a man has mastered the Rig-veda, he says quite loud Om; the same, when he has mastered the Sâman and the Yagus. This Svara is the imperishable (syllable), the immortal, free from fear. Because the Devas entered it, therefore they became immortal, and free from fear.

In plain words: When a student finishes learning any of the Vedas, he says Om out loud. Om never decays, cannot die, and has no fear in it, and because the gods entered it, they became immortal and fearless.