Simplified for Easy Understanding
O best among men [Arjuna], the person who is not disturbed by happiness and distress and is steady in both is eligible for liberation.
Formal:
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.
Simple:
O best among men [Arjuna], the person who is not disturbed by happiness and distress and is steady in both is eligible for liberation.