Bhu, Bhuvas, Suvas, these are the three sacred interjections (vyahriti). Mahakamasya taught a fourth, viz. Mahas, which is Brahman, which is the Self. The others (devatas) are its members. Bhu is this world, Bhuvas is the sky, Suvas is the other world.

In plain words: Bhu, Bhuvas and Suvas are three sacred words spoken in prayer. A sage named Mahakamasya added a fourth word, Mahas, and said it stands for Brahman, the one reality behind everything, and the other three are like its limbs. Bhu stands for this earth, Bhuvas for the sky, and Suvas for the heavenly world.