Bhagavad Gita Verse 45, Chapter 18

sve sve karmanyabhirataha samsiddhim labhate naraha |
svakarmanirataha siddhim yathaa vindanti tachhrunu || 45 ||
 
Engaged in his own duty, each person attains the foremost accomplishment. How one can attain this accomplishment by being content in his duty, that you hear from me.
 
sve : own
karmani : through duty
abhirataha : engaged
samsiddhim : foremost accomplishment
labhate : attains
naraha : person
svakarmanirataha : content in his duty
siddhim : accomplishment
yathaa : how
vindanti : attains
tat : that
shrunu : hear from me
 
The Vedas recognized the system of varna as a means for every individual to realize their potential. They enabled everyone to contribute to society as per their mental makeup and aptitude. Every action performed in accordance with one’s duty yielded a meritorious fruit or punya, and every, and every action that went against one’s duty yielded a demerit or a paapa. The net result of punyas and paapas decided the fate of every individual. One would gain a life that was better or worse than the current one based on their actions.
 
But there is more to the performance of duty than the cycle of action, reaction and rebirth. Shri Krishna says that performance of duty can become the gateway towards liberation. As we have seen earlier, the Gita enables each and every individual, regardless of their occupation or stage in life, to pursue the path of liberation. We do not have to renounce our actions and become sadhus or monks. We just have to keep doing our duty. The result of doing our duty is samsiddhi, perfection, the foremost accomplishment.
 
The ultimate goal of karma yoga is purification of the mind. It cleans the mind of all its impurities in the form of selfish desires created by raaga and dvesha, likes and dislikes. It is this purification of the mind that becomes the foremost accomplishment, samsiddhi, for one who is performing karma yoga. But mere performance of duty will not result in samsiddhi. There is something else needed, which Shri Krishna will reveal in the next shloka.