What is unreal has no being, and what is real has no non-being. The final truth about both of these has been seen by those who perceive the nature of things.
Explanation
Krishna is teaching the grieving Arjuna how to tell what lasts from what does not. The unreal (asat) is what changes and passes away, such as the body. The real (sat) is what never ceases to be, which the following verses identify with the self. Advaita readings take this as a statement about ultimate reality, while other traditions read it as distinguishing the eternal self from changing matter.