श्री भगवानुवाच अक्षरं ब्रह्म परमं स्वभावोऽध्यात्ममुच्यते। भूतभावोद्भवकरो विसर्गः कर्मसंज्ञितः ॥

akṣharaṁ brahma paramaṁ svabhāvo ’dhyātmam uchyate bhūta-bhāvodbhava-karo visargaḥ karma-sanjñitaḥ

The indestructible, the supreme is the Eternal; His essential nature is called Self -knowledge; the emanation that causes the birth of beings is named Action;

In plain words: Krishna answers: the Eternal is the supreme thing that can never be destroyed. Its true nature is the Self. Action is the creative power that brings beings into life.