Bhagwad Gita - Post 212

Verse 23

“Om Tat Sat”: this has been declared to be the triple designation of Brahman. By that were created formerly the Brahmanas, the Vedas and the sacrifices.

The Supreme Being—Brahman, the Absolute—is designated as Om Tat Sat in a threefold definition or description. Knowers of the Vedas, known as Brahmanas, and the Veda mantras, and the yajnas or sacrifices, are all purified and consecrated by the recitation of this mystic symbol Om Tat Sat. 

The threefold description of Brahman as Om, Tat and Sat is always recited in all religious performances—during the study of the Vedas, at the conclusion of sacrifices or yajnas, and whatever rituals that Brahmanas, that is, the knowers of the Vedas, may undertake. Sacrifices (yajna), charities (dana), austerities (tapas), are undertaken by people according to the rules and regulations of the scriptures and as laid down by knowers of Brahman, beginning with the chanting of OM.

OM

Taittiriya Upanisad says “om iti brahman om iti sarvam”, OM is synonymous with the Brahman. OM is understood to be the vibratory cause and spiritual substance of existence. OM is the transcendental sound vibration chanted before commencing every Vedic activity.

The Mundaka Upanisad II.II.VI beginning: om ityevam dhyavatha atmanam states: One should meditate upon OM as representing the Supreme Lord. 

The Chandogya Upanisad beginning sa ya esho anima itad states: That which is the fundamental cause of all existence, which is the spiritual substratum of all existence, which is referred to as Thou art that, is the Brahman which manifests from OM whose source is the Supreme Lord. 

The pranava OM is declared to be the name for the personal aspect of the Supreme Lord. Pranava means first breath and is the transcendental sound vibration OM commencing all Vedic rites. 

TAT 

The term Tat is used in our scriptures to indicate the Eternal Goal, the Changeless and the Ever-perfect. In the famous grand declaration of the Vedas, "Tat-twam-asi”, the term 'Tat' indicates that from which everything has come, in which everything exists and into which everything merges back in the end. 

SAT

Sat means 'Existence’. The "Principle of Existence" functioning through all things perceived, felt, and thought of in our everyday life --- is called Sat.

OM TAT SAT

With respect to the Vedic rituals ,these threefold designations  represent the Brahmanas, the Vedas and Vedic yagnas. 

OM represented the Brahmanas, TAT represents the Vedic scriptures and SAT represents the performance of Vedic yagnas for the satisfaction of the Supreme Lord. Therefore the three words OM TAT SAT are paramount in creation and eternally praiseworthy.

The Eternal Reality, indicated in the triple designation of Brahman, "Om Tat Sat", is the Source from which all castes, the Vedas and the sacrifices proceeded, even at the time of Creation. All super-impositions can only arise from, exist in and disappear into that which is their own substratum.

Love.