Pragâpati brooded on the worlds. From them, thus brooded on, the threefold knowledge (sacrifice) issued forth. He brooded on it, and from it, thus brooded on, issued the three syllables, Bhûh, Bhuvah, Svah.

In plain words: Prajāpati, the creator, thought deeply about the worlds, and from them came the three Vedas. He thought deeply about the Vedas, and from them came three sacred words: Bhūh, Bhuvah and Svah, meaning earth, sky and heaven.