That is called the light of all lights, beyond darkness. It is knowledge, the thing to be known, and the goal reached through knowledge, seated in the heart of everyone.
Explanation
Krishna is still describing Brahman (the ultimate reality), the "knowable" (jñeya) from the previous verses. He says it illumines even the sources of light such as the sun and is beyond darkness, meaning ignorance (tamas). He calls it knowledge (jñāna), the object of knowledge, and what knowledge reaches, dwelling in every heart. Advaita readings take this as the one Self within all, while devotional traditions see the Lord as the inner controller who is distinct from the individual soul.