Chapter 17, Text 23:
oṁ tat sad iti nirdeśo brahmaṇas tri-vidhaḥ smṛtaḥ
“Oṁ tat sat” is advised by the three vedas.
Three vedas here should mean three divisions: karma, jñāna, and upāsana (bhakti). This mantra “Oṁ tat sat” is advised to be used in all three contexts.
Another meaning in these words is:
The three smṛti state that “Oṁ tat sat” indicates the Supreme.
This is because “Oṁ” is a name for the Supreme Being, because “Oṁ” is the seed of all vowels, and thus the seed of all sound – just as the Supreme is the seed of all existence.
brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā
Brāhmaṇas have always used this mantra to comprehend the Vedas and perform Yajña.
This shows that the mantra is to be used for jñāna-yoga (“to comprehend the Vedas”) and for karma-yoga (“to perform Yajña”). It is equally applicable to both because it is upāsana / bhakti. It is an aṅga of bhakti because it is brahman nirdeśa, a name for the Supreme.
tasmād oṁ ity udāhṛtya yajña-dāna-tapaḥ-kriyāḥ
pravartante vidhānoktāḥ satataṁ brahma-vādinām
This is why those who know the Veda always, as a rule, begin every ceremony, charity, austerity, or deed by invoking the word, “Oṁ.”
tad ity anabhisandhāya phalaṁ yajña-tapaḥ-kriyāḥ
dāna-kriyāś ca vividhāḥ kriyante mokṣa-kāṅkṣibhiḥ
The word “tat” indicates that all ceremonies, charities, austerities, and deeds should be done without attachment to conventional results, but with the desire for mokṣa [enlightenment].
“Tat” is simply a pronoun, meaning “that.” But it has this meaning because it follows “Oṁ.” The “that” pronoun points to things. When it follows “Oṁ” it points us to god, to spiritual union with god, mokṣa.
sad-bhāve sādhu-bhāve ca sad ity etat prayujyate
“Sat” is used in the sense of something that is real, and something that is good.
praśaste karmaṇi tathā sac-chabdaḥ pārtha yujyate
[So] use the word “Sat” to incline yourself to good deeds.
yajñe tapasi dāne ca sthitiḥ sad iti cocyate
Become fixed in ceremony, austerity, and charity by pronouncing “Sat.”
karma caiva tad-arthīyaṁ sad ity evābhidhīyate
And, “Sat” indicates that your deeds should be dedicated only to real results.
aśraddhayā hutaṁ dattaṁ tapas taptaṁ kṛtaṁ ca yat
asad ity ucyate pārtha na ca tat pretya no iha
Faithless ceremonies, charity, austerity, and deeds are described as “asat”. Do not aim for them, for they are good neither in this life nor in the afterlife.
“Om tat sat” is a mantra which will significantly help ensure that our approach to the components of religion is not tāmasikī (for ill purpose) or rājasikī (for temporary selfishness).
It means, “Do good deeds (sat = sādhu), and do them for real, eternal goals (sat = real). That is, do them for enlightenment (tat), so that you can come towards the Supreme (oṁ).”
Everyone in every stage of evolution and concept can and should avail of this mantra, by remembering and/or verbally invoking it at the beginning and end of all significant endeavors.