Bhagavad Gita Verse 18, Chapter 4

karmanyakarma yaha pashyedakarmani cha karma yaha |
sa buddhimaanmanushyeshu sa yuktaha kritsnakarmakrit || 18 ||
 
One who sees inaction in action, and action in inaction, he is wise among all people, he is well integrated and accomplishes his actions.
 
karmani : in action
akarma : inaction
yaha : one who
pashyet : sees
akarmani : in in action
cha : and
karma : action
yaha : who
saha : he is
buddhimaan : wise
manushyeshu : among people
saha : he is
yuktaha : a yogi (well integrated)
kritsnakarmakrit : accomplishes all actions
 
Now, having understood the deeper meaning of karma, let us look at the most crucial shlokas of this chapter. Shri Krishna employs his poetic prowess to teach us how to apply the knowledge of karma to our actions. He uses the words karma and akarma (action and inaction) differently in different parts of the shloka, so let us take it part by part.
 
First, let us understand what is meant by "seeing inaction in action". The word action in this phrase refers to activity of any sort: thinking, feeling, working. To be clear, even thinking a thought is action. And the word "inaction" here refers to the constant awareness that the eternal essence, our self, is inactive, and not the doer of action. It means complete detachment from the work and detachment from a sense of agency or doership, because the mind has now attached itself to a higher ideal.
 
So therefore, one who sees inaction in action knows that his every activity is happening out of prakriti or nature. His true identity is the eternal essence that neither does any action not enjoys the result of any action. From a practical standpoint, it refers to the attitude that we have towards our work. It is the difference between a worker who can perform tough tasks and not feel tired, and the worker who feels that every minute of his work is a burden.
 
Next, let us look at the phrase "seeing action in inaction". The word action in this phrase refers to the ego, the notion that "I am the doer" as well as the sense of agency in the work. And the word "inaction" refers to absence of activity. So for example, if someone needs help crossing the road and we think "I won't help him because I will be late for my bus". Behind our absence of helping someone cross the road is a selfish motive. Similarly, if we hold back on admitting a mistake that we had committed, our absence of admitting our guilt is driven by a selfish motive. This is seeing action in inaction.
 
Shri Krishna then goes on to say that one who constantly uses his discrimination to eliminate all sense of doership from every action - that person is wise and is well integrated in the yajna spirit. That person will accomplish any task that he takes up without fail. His success is guaranteed.
 
The following four shlokas describe the attributes of such an integrated person, similar to the characteristics of a wise person from the second chapter.