This chapter continues on the principles of Karma-Yoga from the previous chapters. Sri Krishna tells Arjuna that this greatest secret – the science of Karma-Yoga is inexhaustible  (it is eternal, untouched by time, permanent). Sri Krishna says that HE, HIMSELF had imparted the same science of Karma-Yoga  in a previous era to Vivasvat (Sun), Vivasvat passed it on to his son Manu and Manu gave it to his son Ikshvaku.  Since this wisdom was lost in course of time, HE is revealing the same now to Arjuna who is his devotee and a great friend.