|Chapter 2: Contents of the Gita Summarized|
yam hi na vyathayanty ete
so 'mritatvaya kalpate
O best among men [Arjuna], the person who is not disturbed by happiness and distress and is steady in both is certainly eligible for liberation.
Arjuna may say that despite the theoretical understanding that the soul and the body are distinct, one may still suffer on account of the attachment to the body. Krishna assures him that simply by following one's prescribed duties one is protected from the suffering that comes from acting whimsically based on such attachments.
The Vedic system of prescribed duties is to regulate one's desires in a way that allows for restricted sense enjoyment and gradually purifies one of the same desires. Example is the grahastha (married) ashram, where in one is allowed to enjoy sex life, but in a regulated manner till one becomes naturally detached from the conception of sex. Similarly the ksatriyas are allowed to fight and kill, but only in battle for the protection of others.
Krishna for the first time introduces a spiritual goal, namely liberation. By performing prescribed duties (karma) one gets knowledge (jnana) which is a pre-requisite of liberation (moksha).
Please read the Sanskrit Verse and the Prabhupada's Purport.