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.

Bhagavad Gita Verse 32, Chapter 4

evam bahuvidhaa yajnyaa vitato brahmano mukhe |
karmajaan viddhi taan sarvaanevam jnyaatvaa vimokshyase || 32 ||
 
In this manner, various types of sacrifices have been explained in the Vedas. Understand that all those are born of action; having known this, you will be liberated.
 
evam : in this manner
bahuvidhaa : various types
yajnyaa : sacrifice
vitataaha : have been explained
brahmanaaha : Vedas
mukhe : mouth
karmajaan : born of action
viddhi : understand
taan : those
sarvaan : all
evam : in this manner
jnyaatvaa : having known this
vimokshyase : you will be liberated
 
With this shloka, Shri Krishna concludes the section on practical yajnyas. In this section, we came across many varieties of yajnya. There are several more types of yajnyas in the scriptures such as pilgrimages, how to perform poojas during certain festivals, chanting of japas and so on. With the guidance of a teacher, we can choose the one that works best for us and follow it diligently.
 
Having concluded this section, Shri Krishna now makes a very important point. He says that while performing these yajnyas, we should never forget that any yajnya is ultimately an action. We saw earlier that actions are performed by the gunaas of prakriti. And anything that is related to gunaas is distinct from the eternal essence. So, how can yajynas move us closer to liberation if they are in the realm of gunaas?
 
Another way of looking at this issue is as follows. Anything that is a result of action will always be finite by definition. We are looking for realization of the eternal essence which is infinite. How will yajnyaas, born out of finite action, make us obtain the infinite eternal essence?
 
The key point to understand here is that action does not yield realization, only knowledge can do so. Performance of yajnya is a preparatory step towards realization, just like we prepare for sleep in the night. We can lie down on the bed, we can drink a cup of hot milk, we can turn off the light and so on. But the onset of sleep happens on its own. Similarly, even if we perform yajnyaas, realization will happen when we gain the knowledge when we are not the doer or enjoyer of any actions.