Purport

In Above Shloka Bhagvan Shri Krishna explains the limitation of external repression. The internal nature of humans is more important as every human in this living world behaves as per his internal nature. Acts of a good person is good and that of bad is bad. Therefore correcting the internal nature one can correct his external actions. Similarly looking at the external actions one can examine his one internal nature. If there are any actions that produce miseries and harm to other beings, one should immediately realize that there is something wrong with his internal understanding. He should hence try to correct himself.

There are many ways to purify the internal nature. In above Shloka Bhagvan Shri Krishna already explained that having faith in Bhagvan Shri Krishna and following his words is one of the ways of purification.