समो भूत्वा समत्वं योग उच्यते .....

योगस्थः कुरु कर्माणि सङ्गं त्यक्त्वा धनञ्जय
सिध्यसिध्योः समो भूत्वा समत्वं  उच्यते । । ……… २:४८

हे धनञ्जय ! तू आसक्तिको  त्यागकर तथा सिद्धि और असिद्धि में समान बुध्दि वाला होकर योगमे स्तिथ हुआ कर्तव्यकर्मों को कर , समत्व ही योग कहलाता है।

Be steadfast in yoga, O Arjuna. Perform your duty and abandon all attachment to success or failure. Such evenness of mind is called yoga

Those who wanted to learn yoga should read this sloka several times and understand it better before joining any yoga classes so that one would know at least what to attain through it, most of the people think  yoga means breathing exercise which can keep their body fit. Whole gyana told by the Bhagvan krishna  to arjuna is that this body is अनित्यम , अशस्वतो , but utilize this body for doing Dharma ,To know that, higher knowledge, know that नित्यं  which is seated in this body. 

Hence Practice equanimity in both the situations whether you mastered some thing or you failed drastically(सिद्धि - असिद्धि ) be calm continue do your duties believing the fruits as milestones, instill faith in lord, until finally this individual consciousness(जीव ) meets universal consciousness(ब्रह्म ).

commentaries of various saints

Yoga is the science of the individual consciousness attaining communion with the Ultimate Consciousness. 

  Being established by this yoga perform actions relinquishing attachment, motivation for rewards and depend solely upon the mercy of the Supreme Lord in all one's activities. Totally unconcerned about success or failure, attainment or non-attainment resultant surrender in righteousness all actions as an offering unto the Supreme Lord. This is the eternal path called yoga by the wise as it consists of fixed concentration of the mind. Yoga, equanimity is the methodology encouraged. Relinquishing desire and attachment to the rewards of actions while steadfastly remaining equipoise in either success or failure. This verily is what is meant by equanimity.The meaning of the word sangam is attachment. Attachment to what? The rewards of one's fruitive actions.

Hence What should one do? Lord Krishna states here to remain balanced and equipoised. Discard the mentality of being attached. Be balanced in either victory or defeat and perform your duties.कर्माणि  refers to duties the plural usage of duties indicates various obligatory and occasional duties; Thus Yoga is explained by Lord Krishna Himself as being in the state of equanimity. 

 Actions performed with fruitive motivations (Intentions ) are far inferior to actions performed as a matter of duty.when the mind of those whose actions are performed as a matter of duty are not disturbed or unbalanced by delusions of rewards gained or lost. The activities performed in spiritual intelligence removes all suffering in the world and leads to liberation from the material existence. Contrarily activities performed for the acquisition of rewards assuredly results in suffering and affliction in the material existence.फल-हेत्वा Phala-hetavah are those who sole motivation is dictated be the prospects of reward. It should be understood that such living entities are krpanah or pitiable due to the fact but acting in this way they are imprisoned in the material existence unable to attain their spiritual nature.

Definition of EQUANIMITY

1:  Evenness of mind especially under stress (nothing could disturb his equanimity)

2:  Right disposition :  balance