"Om Tat Sat" is the eternal sound-pranava. "Om Tat Sat" represents the unmanifest and absolute reality. The word "reality" here means total existence. God, reality, existence, Para Brahman and the Absolute, are all synonymous terms pointing to one Supreme Being.
In the seventeenth chapter of the Bhagavad Gita, from verses 23 to 28, Lord Krishna discussed the meaning and importance of the mantra Om Tat Sat. He said that Om Tat Sat is actually a threefold name of the Supreme Soul with which, at the start of the universe, the Brahman, Vedas and Yajna were made.
The words “Om Tat Sat” have been declared as symbolic representations of the Supreme Absolute Truth, from the beginning of creation. From them came the priests, scriptures, and sacrifice. (17.23)
Therefore, when performing acts of sacrifice, offering charity, or undertaking penance, expounders of the Vedas always begin by uttering “Om” according to the prescriptions of Vedic injunctions. (17.24)
Persons who do not desire fruitive rewards, but seek to be free from material entanglements, utter the word “Tat” along with acts of austerity, sacrifice, and charity. (17.25)
The word “Sat” means eternal existence and goodness. O Arjun, it is also used to describe an auspicious action. Being established in the performance of sacrifice, penance, and charity, is also described by the word “Sat.” And so any act for such purposes is named “Sat.” (17.26-17.27)
O son of Pritha, whatever acts of sacrifice, charity, or penance are done without faith, are termed as “Asat.” They are useless both in this world and the next. (17.28) 
This word represents all the actions & duties which are performed with truthfulness and related to the eternal truth. It is also said the everything present in yajna, tapas & dana are considered as "Sat", as well as actions meant solely for the satisfaction of the Supreme.
- "Bhagavad-Gita: Chapter 17, Verse 23". www.bhagavad-gita.org.
- Mukundananda, Swami. "Chapter 17 – Bhagavad Gita, The Song of God – Swami Mukundananda". www.holy-bhagavad-gita.org. Retrieved 2022-03-01.
- "Bhagavad-Gita: Chapter 17, Verse 25". www.bhagavad-gita.org.
- "Bhagavad-Gita: Chapter 17, Verse 26,27". www.bhagavad-gita.org.