Bhagavad Gita 17.23तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः।ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ॥

Om Tat Sat is the root of the entire universe. Om is the Akshara Brahman. Tat means Thath, the indefinable. Sat means Reality.Para Brahman, that Supreme Being, the abiding place of all that lives and moves, is beyond name and class. The Vedas have ventured to give a name to Him. A new born child has no name but no,receiving one he will answer to it. Men who are troubled by the afflictions of this world run to the Deity for refuge and call Him by the name. When Brahman is invoked through the name that which is hidden is revealed to the aspirant.These three words have a divine power of their own. The vibrations that they produce in one are such as to arouse the latent divinity and also to secure the necessary response from the Cosmic Being Whom they connote.When a sacrificial rite or the like is found defective, it will be rendered perfect by the utterance of the powerful Mantra Om Tat Sat or one of the three designations in the end. With Om or Om Tat Sat all acts of sacrifice, study of sacred scriptures, spiritual discipline and meditation are commenced. If the doer of sacrifices remembers either of these Mantras all obstacles that stand in the way of success of the sacrifices are removed.Om Tat Sat has been declared to be the triple designation of Brahman in the Vedanta by the knowers of Brahman. The power of creation that lies in the Creator emanates from this Mantra