क्षेत्रज्ञं चापि मां विद्धि सर्वक्षेत्रेषु भारत। क्षेत्रक्षेत्रज्ञयोर्ज्ञानं यत्तज्ज्ञानं मतं मम ॥

kṣhetra-jñaṁ chāpi māṁ viddhi sarva-kṣhetreṣhu bhārata kṣhetra-kṣhetrajñayor jñānaṁ yat taj jñānaṁ mataṁ mama

Understand Me as the Knower of the Field in all Fields, O Bhârata. Wisdom as to the Field and the Knower of the Field, that in My opinion is the wisdom.

In plain words: Krishna says he himself is the Knower present in every body. He says that knowing the difference between the body and the one who knows it is real wisdom.