Then comes this breath (of life) in the mouth. They meditated on the udgîtha (Om) as that breath. When the Asuras came to it, they were scattered, as (a ball of earth) would be scattered when hitting a solid stone.

In plain words: Next the gods meditated on Om as the breath of life that moves through the mouth. When the Asuras (demons) attacked it, they were smashed apart, like a lump of clay thrown against a hard rock.