Seated there on that seat, making the mind one-pointed, with the workings of mind and senses held in check, he should practise yoga for the purification of the self.
Explanation
Krishna is continuing his instructions to Arjuna on how to prepare for meditation; the previous verse described the seat, and here he describes the practitioner's inner state. The aim of the practice is the purification of the self (ātma-viśuddhi), and yoga here means disciplined meditation. Some commentators take the "self" as the mind and heart, others as the inner self (ātman) freed from impurity; both agree the goal is inner clarity.