Simple Translation: O thick-haired (Arjuna), I am the Ātmā (soul) abiding in the heart of all beings. I am also the beginning, the middle, and the end of all beings.
Jyotiṣa Notes: The sun represents the soul and the life force in all living beings. It defines the inner nature as well as reveals the form, of all beings. It is the source of creation of everything manifest, as well as the source of their continued existence (sustenance) and finally their dissolution when the sustenance ends. These activities are defined by the three guṇa rajas-sattva-tamas respectively and the guṇa avatāra Brahma, Viṣṇu and Rudra. The three sounds A-U-M define the source of each of these states of every being as well as the source of their creation, sustenance and dissolution.