Agriculture, commerce and tending of cattle—the knowledge of these three professions, O noble Sire, is the practical science. Agriculture is the means of subsistence to the cultivators, buying and selling to the traders, and tending of cattle is our subsistence. The practical science has thus been divided into three branches. The object, that is cultivated by any one, should be to him, his chief deity—he must worship that, for that is his benefactor.

In plain words: Krishna says there are three ways of making a living: farming, trading and raising cattle. Farmers live by farming, traders by trade, and the cowherds live by tending cows. So whatever actually supports a person should be what they honor most, because it is what helps them.