न प्रहृष्येत्प्रियं प्राप्य नोद्विजेत्प्राप्य चाप्रियम्। स्थिरबुद्धिरसम्मूढो ब्रह्मविद्ब्रह्मणि स्थितः ॥

na prahṛiṣhyet priyaṁ prāpya nodvijet prāpya chāpriyam sthira-buddhir asammūḍho brahma-vid brahmaṇi sthitaḥ

With Reason firm, unperplexed, the knower of the Eternal established in the Eternal, neither rejoiceth on obtaining what is pleasant, nor sorroweth on obtaining what is unpleasant.

In plain words: Krishna says that a person who truly knows God has a steady mind and is not confused. They do not get overexcited when good things happen or crushed when bad things happen.