Aum is the basic mantra of all Sanskrit mantras. It refers to A(jagrat), U(swapna), and M(sushupti). The three regions refer to the three planes of existence, physical, astral and casual. The silence which follows each utterance of Aum suggests the state of Absolute Being, or Brahman, which is the source and end of everything.
There are three types of mantras : mantras which gives tamsic, rajsic, and sattvic qualities. Gayatri Mantra belongs to the third kind. Gayatri Mantra is the most sacred of all mantra in the vedic texts and is often referred to Moola Mantra. Gayatri = Gaya + tri ( singer + protectress), meaning, 'She who protects or saves the singer.'
OM BHUR BHUVAH SVAH TAT SAVITUR VARENYAMBHARGHO DEVASAYA DHIMAHI DHIYO YO NAH PRACHODAYAT
Hear the Enchanting Mantra
... through the absolute