Bhagavad Gita Verse 13, Chapter 4

chaaturvarnyam mayaa srishtam gunakarmavibhaagashaha |
tasya kartaaramapi maam viddhyakartaaramavyayam || 13 ||
 
Four classes have been created by me, based on the division of guna and action. Even though I created them, know me as the non-doer and imperishable.
 
chaatur : four
varnyam : classes
mayaa : by me
srishtam : created
guna : guna
karma : action
vibhaagashaha : division
tasya : that
kartaaram : being the doer
api : even though
maam : me
viddhi : know
akartaaram : non-doer
avyayam : imperishable
 
Earlier, Shri Krishna mentioned that humans cannot avoid action at any cost. So how can we achieve liberation from bondage while still performing action? To that end, Shri Krishna advises us to act per our svadharma. By efficiently performing svadharma, we can liberate ourselves while performing actions. This is the "why" of karmayoga.
 
Knowing that one should perform one's svadharma or duty in this world is core to karmayoga. But how does one know what is one's svadharma? Shri Krishna addresses this point briefly in this shloka. He says that human beings are categorized into four classes or varnaas. These classes are based on the 3 gunaas, and the corresponding action that each guna prompts us to do.
 
The three gunas and their corresponding actions are as follows. Rajas is expressed as as activity or agitation. Tamas is expressed as lethargy or laziness. Sattva is expressed as knowledge and peace. So we have to perform self-analysis in order to understand how the gunaas behave within us and in what proportion to each other.
 
We will find that we will fall into one of these four categories. A brahmana who is predisposed to gaining knowledge, faith, sharing knowledge will usually have a prominence of sattva. A kshatriya who demonstrates courage, likes to organize and protect people, face challenges, take risks, try new things will have a prominence of sattva and rajas. A vaishya who likes to be creative and produce something will have a prominence of rajas and tamas. A shudra who likes to execute tasks but requires a lot of motivation will have prominence of tamas.
 
As we can tell, this shloka was heavily misinterpreted to support the incorrect notion that varna is determined by birth. There is no such thing. Just like we have career counselling in modern times, the Gita offered a scientific manner of selecting a career that is suitable for oneself both from a practical perspective, and also from a karma yoga perspective.
 
In the second part of the shloka, Shri Krishna reminds us that although he has set up this classification of varnaa, he is not the doer even in this act. It is maayaa alone that is acting in this world, whereas he is only the witness to its actions. We can think of ourselves as playing different parts in a cosmic play. Each part is different based on our svadharma.