Oṃ bhūr bhuvaḥ svaḥ
Om: The sacred word, word of creation, first word, word of god; (Equivalent of Holy Ghost in trinity) usually used at the beginning of a hymn;
Bhur: The physical plane of existence (which is of the nature of 5 elements)
Bhuvah: The astral plane of existence (which is of the nature of subtle elements)
Svah: The causal plane or celestial plane. (plane where the existence is as subtle as ideas or notions, from which creation happens).