Brahman is the imperishable, the supreme; one's own inner nature is called the self (adhyatma). The creative outpouring that brings beings and their states into existence is called action (karma).
Explanation
Krishna begins answering the questions Arjuna asked at the start of the chapter: what Brahman, the self (adhyatma) and action (karma) are. Brahman (the ultimate reality) is the imperishable; one's own inherent nature (svabhāva) is called adhyatma, which Advaita readers take as Brahman present as the inner self and other schools as the self's own embodied nature. Karma here is visarga, literally a 'sending forth' or offering, the creative act that brings beings into being; commentators differ on whether this means ritual offering or the wider creative process.