Bhagavad Gita Verse 12, Chapter 18

anishtamishtam mishram cha trividham karmanaha phalam |
bhavatyatyaaginaam pretya na tu sannyaasinaam kvachita || 12 ||
 
The three fold fruit of action, undesirable, desirable and mixed, goes after death to the atyaagees, but never to the sanyaasis.
 
anishtam : undesirable
ishtam : desirable
mishram : mixed
cha : and
trividham : three fold
karmanaha : actions
phalam : fruit
bhavati : goes
atyaaginaam : atyaagees
pretya : after death
na : not
tu : but
sannyaasinaam : to a sanyaasi
kvachita : ever
 
Shri Krishna elaborates on the theory of karma in this chapter. Every action has a reaction, and this reaction is also known as the phala, result or fruit of the action. When we cross the road to catch a bus, we could get one of three results. We could catch the bus, we could miss the bus, or we could get into a packed bus with no room for seating. The result could be desirable, undesirable or mixed. This is a law of the universe, no different than the law of gravity. The fruit always comes to the performer of the action, with the same certainty as the bill collector comes to collect a debt, per the words of Sant Jnyaneshwar.
 
Any fruit that is found in nature contains seeds. These seeds can create trees that can generate many more fruits. Similary, every fruit that results from an action has the potential to create innumerable desires that result in even more actions. This infinite chain of action, reaction, fruit, desire and action is nothing but bondage or samsaara. One who is an atyaagi, one who has not given up attachment to fruits of action, is bound in this chain. By practicing karmayoga, by giving up attachment to the fruit of action, we can break this chain.
 
Now, let us proceed to the next phase of karma yoga. As long as we hold the notion that we are performing all actions, we cannot proceed further in the journey of a seeker. Performance of action is falsely superimposed on the eternal essence which is beyond all action, beyond all change. The second chapter made it very clear that there can be no change in the aatmaa, the eternal essence. Unless we fully internalize the notion that our "I", our self does not perform action, we will remain entrenched in samsaara. Shri Krishna says that only a sanyaasi, only one who sees that Prakriti really performs actions, can destroy current, past and future fruits of action. This topic is elaborated in the following shlokas.