योगस्थः कुरु कर्माणि सङ्गं त्यक्त्वा धनञ्जय। सिद्ध्यसिद्ध्योः समो भूत्वा समत्वं योग उच्यते ॥

yoga-sthaḥ kuru karmāṇi saṅgaṁ tyaktvā dhanañjaya siddhy-asiddhyoḥ samo bhūtvā samatvaṁ yoga uchyate

Perform action, O Dhananjaya, dwelling in union with the divine, renouncing attachments and balanced evenly in success and failure: equilibrium is called yoga.

In plain words: Krishna tells Arjuna (called Dhananjaya, winner of wealth) to act while connected to God, letting go of attachment and staying the same in success or failure. This steady balance is what yoga means.