This is the concluding Episode of the Kathopnishad, the dialogue between Nachiket and Yamraj.

Yamraj explains to Nachiket that we need use our senses and mind to look within to know our soul. Once we understand the soul, we know that God and the soul are one and the same thing, like light and shadow, that cannot be separated. 

Yamraj then explains that in this life, whatever work we have done and the knowldege we have gained, through our body and mind, according to that karma, the soul needs to take re-birth after death. He explains that till we don't look within to understand the soul, our soul keeps taking re-birth. But, once we understand the soul, we attain nirvana/ moksh/ mukti, and then the soul does not take re-birth as it merges with the God/ Divine.