आत्मौपम्येन सर्वत्र समं पश्यति योऽर्जुन। सुखं वा यदि वा दुःखं सः योगी परमो मतः ॥

ātmaupamyena sarvatra samaṁ paśhyati yo ’rjuna sukhaṁ vā yadi vā duḥkhaṁ sa yogī paramo mataḥ

He who, through the likeness of the Self, O Arjuna, seeth equality in everything, whether pleasant or painful, he is considered a perfect Yogî.

In plain words: Krishna tells Arjuna that the perfect yogi feels others' joy and pain as if it were his own. Because he sees himself in everyone, he treats all beings equally, whether in pleasure or in pain.