The two words kartṛ or kartā (‘the doer’) and bhoktṛ or bhoktā (‘the enjoyer,’ ‘the experiencer’) are commonly used in the Hindu philosophical works as applied to the jīvātman (the individual soul). Though the ātman (the Self or the soul) is ever free, how he becomes bound as the jīvātman due to association with avidyā or ajñāna (nescience) or prakṛti (nature) is a question that can never be answered satisfactorily since our mind which raises this question is itself the product of that nescience.
Once the existence of the jīvātman which is a matter of our common experience is conceded, its corollaries too can be accepted. He is hence both the kartṛ and the bhoktṛ of actions. He experiences the results of all actions done by him. It is this fact of his being a doer and the experiencer of actions and their fruits that results in his bondage leading to rebirth.
Meditation on the ātman as his true Self, which is neither the kartṛ nor the bhoktṛ will ultimately lead to mokṣa or liberation from transmigration.