Krishna enters a new section. Both Prakriti and Purusha, he says, are beginningless. Neither was born before the other. Both have always been.
And the modifications we observe — pleasure, pain, desire, aversion — and the three gunas (sattva, rajas, tamas) — all of these arise from Prakriti. The Purusha, the soul, stands beyond them.
This verse lays the foundation of Sankhya philosophy. Prakriti and Purusha are the two fundamental realities whose interaction produces the entire manifest world. The Gita accepts this framework while adding devotion to the picture.
In the Gita Press edition, this is the twentieth shloka. The Prakriti-Purusha analysis runs from here through several verses.