Krishna explains to Arjuna that as a warrior, it is his duty to fight in the war and protect his people, even if it means killing his own relatives. Krishna stresses the importance of fulfilling one's dharma, or duty, in life and that this is the path to spiritual liberation.
Krishna also teaches Arjuna the concept of detachment and how one should perform their actions without being attached to the results. He emphasizes that one should do their duty without worrying about the outcomes or rewards, and focus on the present moment.
Overall, the most important thing that Krishna told Arjuna is to follow one's duty or dharma and perform actions without attachment to the results. This concept is essential in Hindu philosophy and is applicable to many aspects of life, including work, relationships, and spirituality.