Bhagavad Gita Verse 22, Chapter 4

yadricchaalaabhasantushto dvandvaateeto vimatsaraha |
samaha sidhhavasidhhau cha kritvaapi na nibhadyate || 22 ||
 
Content in whatever comes of its own accord, transcending duality, free from envy, balanced in success and in failure, (he) is not bound even when performing actions.
 
yadricchaa-laabha-santushtaha : content in what comes of its own accord
dvandva-ateetaha : transcending duality
vimatsaraha : free from envy
samaha : balanced
sidhhau : in success
asidhhau : in failure
cha : and
kritvaa : performing actions
api : even when
na : not
nibhadyate : bound
 
While progressing in our project, we may encounter several situations, some of which we expected and some that we did not. Shri Krishna guides us on how to deal with these situations in this shloka.
 
By definition, karma yoga inspires us to work relentlessly. The output of our project may be sometimes favourable, and sometimes not. The follower of karma yoga knows that he is one of the many variables that determine the output of his work, and is therefore content with whatever comes his way. He does not let unfavourable outcomes impact his level of work. This is the "prasaada buddhi" that we say earlier.
 
Favourable and unfavourable, success and failure, heat and cold, praise and criticism - this is duality. It arises because our mind tends to label one aspect of nature as positive, and reject the other as negative. But the follower of karma yoga knows that nothing is absolutely good or bad. It is all part of Ishvaraa and therefore remains equanimous. This is nothing but "samatva buddhi".
 
Our mind has a natural tendency to compare ourselves with others like us. If it perceives us "better" than others, it generates pride. If it perceives us "inferior" to others, it generates envy. The follower of karma yoga knows that ultimately we are all part of the same higher ideal - Ishvaraa. Any envy generated in the mind only strengthens the ego. So he never lets envy distract him  from his work. It is a quality that he does not encourage.
 
So the refrain here is that such a follower of karma yoga will continue to perform actions in this world, yet remain unattached because he never lets external situations destabilize his equanimity.