This understanding has been given to you in the teaching of Sankhya. Now listen to it as it applies to yoga, O Partha. Armed with this understanding, you will cast off the bondage of action.
Explanation
Krishna turns from the teaching on the self, which he has just given under the name Sankhya (analytical knowledge of the self and reality), to the practical path of yoga (disciplined action). Here 'understanding' (buddhi) means steady, discerning judgment. He promises that acting with it frees Arjuna from the bondage of action (karma-bandha), meaning the way deeds tie a person to their results. Some traditions read 'Sankhya' as the philosophy of knowledge, others as simply 'reasoned explanation'; either way, the verse introduces the teaching of karma yoga.