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 10, Chapter 3

sahayajnah prajaah srishtvaa purovaacha prajaapatih |
anena prasavishyadhvamesha vostvishtakaamadhuk || 10 ||
 
In ancient times, Prajaapati created humanity along with yajna. He said "through this (yajna) let everyone prosper,  and may it become your fulfiller of wishes".
 
sahayajnah : with yajna
prajaah : humanity
srishtvaa : created
puraa : in ancient times
uvaacha : said
prajaapatih: prajaapati
anena : through this
prasavishyadhvam : may you prosper
eshah : this
vah : for you
astu : become
ishtakaamadhuk : fulfiller of wishes
 
A common question for many of us is: "I understand the concept of selfless dedication. However, how do I know that I will be able to fulfill my material needs if I give up caring for the fruits of action altogether?" Shri Krishna has the answer : perform actions with selfless dedication or yajna, and your material needs will be provided for automatically.
 
If we work for a corporation, we do several different things everyday to fulfill our job responsibilities: respond to emails, attend meetings, draft project plans, make excel documents and so on. Do we ever think : "how much money will I get paid for opening this email? how much money do I get for attending this meeting?" If we have done our job right, the results will automatically come to us in the form of a monthly salary.
 
Shri Krishna explains that yajna is embedded within the fabric of humanity since time immemorial. It is a universal law. Once invoked, that law provides us with all that we need in this world. It becomes the "fulfiller of wishes". And the key to invoke that universal law is to perform selfless action dedicated to a higher ideal.
 
This is the essence of karmayoga.
 
Footnotes
1. Some commentators interpret the word "isthakaamadhuk" to mean "kaamadhenu", the mythological cow that grants any wish asked of her. But even with this interpretation, the essence of this shloka remains unchanged.

Bhagavad Gita Verse 3, Chapter 3

Shree Bhagavaan uvaacha:
lokesmindvividhaa nishthaa puraa proktaa mayaanagha |
jnyaanayogena saankhyaanaam karmayogena yoginaam || 3 ||
 
Shri Bhagavaan said:
In this world, since time immemorial, I have created a two-fold path, O sinless one; the yoga of knowledge for contemplative individuals, and the yoga of action for yogis.
 
loke : world
asmin : this
dvividhaa : two-fold
nishthaa : path
puraa : since time immemorial
proktaa : spoken
mayaa : by me
anagha : O sinless one
jnyaanayogena : the yoga of knowledge
saankhyaanaam : for contemplative individuals
karmayogena : the yoga of action
yoginaam : for yogis
 
Earlier, we saw that Arjuna wanted to know which of the two paths, i.e. the yoga of action and the yoga of knowledge, is superior. Shri Krishna addresses that question here. According to him, none of the two is either superior or inferior.
 
Sant Jnyaneshwar provides an example to show that none of these paths are superior or inferior: like a west-flowing river and an east-flowing river that both meet the same ocean, both paths are same.
 
Now, even thought the paths are complementary, there is a subtle difference between them. Let us explore this difference. Consider a child who is in the first grade. Even if he wants to, he cannot jump straight to a masters degree. He would have to finish high school, then college, and only then will he be ready for his masters degree.
 
In the same way, most of us are at a stage in life where we are active and want to make a contribution to the world. For such yogis or active individuals, the path of action is the most appropriate. Once we reach a stage where we exhaust all of our desires, we can transcend the path of action to pursue the path of knowledge, just like we transcend college education to pursue our masters degree.
 
I used to know a classmate from school that was a math prodigy. He could solve advanced calculus equations in sixth grade. The teachers used to check with him whether they were teaching sixth grade math correctly, such was his prowess. He did not need to go through elementary math, he was ready for his math PhD even at that young age.
 
Similarly, there are a few rare individuals who are at an advanced stage of their spiritual journey. They have eliminated most of their desires. Only such rare contemplative individuals can directly pursue the path of knowledge. For most of us, the path of action is the way to go. And karmayoga, or the path of action, is the main subject covered in this chapter.
 
On a different note, we see that Shri Krishna is hinting that he is something more that the cowherd son of Vasudeva here. We know this because he mentions that he himself has created this two-fold path for realizing the eternal essence. This has not slipped past Arjuna, he will ask this question at the right time.