Bhagavad Gita Verse 23, Chapter 17

om tatsaditi nirdesho brahmanastrividhaha smritaha |
braahmanaastena vedaashcha yajnyaashcha vihitaahaa puraa || 23 ||
 
Om, Tat, Sat, this has been declared as the triple indicator of the eternal essence. By that were created the braahmanaas, the Vedas and the sacrifices, long ago.
 
om : om
tat : tat
sat : sat
iti : in this manner
nirdeshaha : indicator
brahmanaha : eternal essence
trividhaha : triple
smritaha : declared
braahmanaaha : braahmanaas
tena : by that
vedaaha : Vedas
cha : and
yajnyaahaa : sacrifices
cha : and
vihitaahaa : created
puraa : long ago
 
Arjuna began this chapter with a question to Shri Krishna: How can we determine the texture of our faith, as to whether it is saattvic, raajasic or taamasic? Shri Krishna replied that we need to infer this from our actions and the attitude behind our actions. He then showed us the three types of food, sacrifice, penance and charity. The takeaway for us is that we should slowly stop performing raajasic and saatvic actions, and increase our saattvic actions. But even if we manage to perform only sattvic actions, there will always be the chance of some error creeping in.
 
Here, Shri Krishna gives us a formula, a technique to remove errors and defects in saattvic actions. This purifier is the combination of three words - Om, Tat and Sat. All three words are indicators of the eternal essence, of brahman. In fact, each of these words has been used in the Vedas to refer to the eternal essence. Om comes up in the Rig Veda. Tat and Sat come up in the Chhaandogya Upanishad. The word Sat is also used in the second chapter of the Gita to refer to the eternal essence. Note that the eternal essence is beyond name and form. These words are only pointers.
 
Next, Shri Krishna gives us the heritage of this purifier. Prajaapati, Lord Brahma, created the universe along with the sacrifice and the Vedas, as we have seen in the third chapter. He also created braahmanaas, who performed rituals and sacrifices with faith in the Vedas. These braahmanaas would always use Om Tat Sat to purify their sacrifices, to remove any defects in the rituals. Now, per the teaching of karma yoga, whenever we perform saattvic actions, they automatically become a sacrifice. Therefore, even we can use Om Tat Sat to purify our saattvic actions.