In the producing of effects and their causes, nature (prakriti) is said to be the cause; in the experiencing of pleasure and pain, the purusha is said to be the cause.
Explanation
Krishna is explaining the two basic principles of the field and its knower: nature (prakṛti), the material source of everything, and the conscious self (puruṣa). Nature is the agent behind everything that gets produced, including bodies, senses and actions, while the puruṣa is the one who experiences pleasure and pain (bhoktṛtva). Commentators differ on 'kārya-kāraṇa': some read it as effects and their causes, while others follow a variant text (karaṇa) and take it as the body and its instruments, the senses; traditions also differ on how the puruṣa relates to nature, but all take the verse to divide the roles into acting and experiencing.