Srimad BhagavadGita

In the previous śloka, Kṛṣṇa instructed Arjuna that he must renounce the desire for the chief fruit of his action, eg: desire on the chief reward of heaven obtained on account of fighting a righteous war. In this śloka, Kṛṣṇa is instructing Arjuna that he must accept the intermediate rewards which will inevitably come his way like victory or defeat in a war, etc, in the same manner. Therefore, this śloka is an elaboration of the previous śloka which discussed the renunciation of rewards of the actions.

Kṛṣṇa says, "O Arjuna, giving up attachment towards kingdom and relatives, perform your duties by being established in the yoga of treating alike achievement and non-achievement of victory. This equipoised state in both is called yoga."  

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saṇgaṃ tyaktvā (Not holding on to purposes like kingdom, relatives, etc.) When there is a strong hold on purposes like kingdom, relatives, etc, one of the conditions for karma yoga called 'renunciation of rewards of actions' is not satisfied. Karma yoga performed in this manner becomes invalid. If he has attachment towards his relatives, he will be unable to even fight the war against them, leaving no scope for the execution of karma yoga at all. Therefore Kṛṣṇa instructs Arjuna that he should not hold onto them.

karmāṇi kuru (you must perform your varṇāśrama duties which include fighting the war)

Here a doubt might arise: Performance of his varṇāśrama duty like fighting the war will inevitably deliver the reward of victory or defeat whether he desires it or not and these will lead to happiness and sorrow respectively. Won't these happiness and sorrow cause hurdles in the practice of karma yoga?

Clarification: Kṛṣṇa clarifies the doubt with the instruction yogasthaḥ kuru (being established in yoga perform your varṇāśrama duties). It means that the duties should be performed by remaining established in yoga. When performed in this manner there will be no hurdles. 

Q: What does it mean to perform varṇāśrama duties while being established in yoga?

A: siddhy-asiddhyoḥ samo bhūtvā kuru This instruction by Kṛṣṇa answers the above question. It means that the varṇāśrama duties must be performed treating alike lābhālābhau jayājayau B.G 2.38 (achievement or non-achievement of victory or wealth) which are the intermediate rewards for duties like fighting a war, etc, This is what it means by performing actions being established in yoga.

Q: Does the word yoga actually have such a meaning as stated above?

A: samatvaṃ yoga ucyate This is the answer. It means that the learned people call the ability to have an unwavering mind irrespective of whether victory, etc, are accomplished or not, as yoga. Pātanjali yoga sūtras and other such śāstras too use the term yoga to denote the balanced unwavering state of the mind. yoga-kṣemaṃ vahāmyaham B.G 9.22, here the word yoga has a different meaning though. However the earlier meaning suits the present context and hence it is taken. In places like paṇḍitāḥ sama-darśina B.G 5.18, the word samatvaṃ has a different meaning, but based on the context established by the earlier words 'siddhy-asiddhyoḥ samo bhūtvā', an equipoised mind is the apt meaning here.

dhanañjaya (one who has conquered wealth) Kṛṣṇa's intention behind addressing Arjuna as Dhanañjaya here is, 'You who has earned this name must acquire this great wealth of having an equipoised mind too.'