He brooded over the threefold knowledge (the three Vedas), and from it thus brooded on he squeezed out the essences, the sacred interjection Bhûs from the Rik verses, the sacred interjection Bhuvas from the Yagus verses, the sacred interjection Svar from the Sâman verses.

In plain words: Then he thought deeply about the three Vedas, the holy scriptures, and drew out their essence. He got three sacred words: Bhuh from the Rig verses, Bhuvah from the Yajur verses, and Svah from the Sama verses.