Purport

This is to be noted that this is one of the Shlokas of which has been misquoted by some Anti-Hindu forces like Jakir Naik.

This is one of the most important Shlokas of Shrimad Bhagvad Gita and explains the core of Karma Yoga. The key to understanding this Shloka lies in understanding the meaning of Dharma. Dharma is a word that is used for a wide range of subjects. One of the very root meaning of the Dharma is the original nature of living and non-living entities.
When applied for non-living beings it explains the nature of that object, As Dharma of water is to clean and flow. The Dharma of fire is to burn.

When applied for a living being, Dharma explains internal nature. Like music is the dharma of Musician. To paint is the dharma of the painter. But Dharma also has another important meaning and that is that the duty. As the Dharma of the teacher is to teach without any discrimination. The Dharma of the doctor is to save lives. The dharma of the soldier is to fight for the protection of society. Etc.

Above shloka holds good for both meanings, the internal nature and duty for humans. The Shloka says that every human should perform such a task that suits his internal nature even if the task has some faults rather than trying to do some other task which might be giving more returns but does not match his internal nature. For example, a singer is best suited to choose the task of singing even if it is not as rewarding as sports because that is what he can do better. If a person who has an inclination towards music tries to go to sports may fail drastically. Worst is that doing something which does not match the nature of human takes away all the joy of doing the task.

Through this Shloka Bhagvan Shri Krishna hence guides us to perform our natural duties which suit our internal nature rather than trying to copy somebody else.