In this verse Lord Krishna advises Arjuna how to fight the battle he is trying to avoid. Life in the world functions by ego, attachment and desire, which gives the rise to the idea of likes and dislikes. In this way the mind starts identifying all experiences in the world in terms of opposites, such as pleasure and pain, gain and loss, victory and defeat... Here, Sri Krishna is saying that if Arjuna has neither desire for heaven nor for sovereignty over the earth, then he should achieve equanimity of the mind. With equanimity of the mind one can achieve success in the war of life. Without it, one cannot remain unaffected by the pairs of opposites and will be continually tossed about by the waves of egocentric likes and dislikes.

Srimad Bhagavad Gita, Ch. I-VI, 2013