Sri Aurobindo translates Katha Upanishad, First Cycle: Third Chapter, Verses 1-2: “Yama speaks: ‘There are two that drink deep of the truth in the world of work well-accomplished: they are lodged in the secret plane of being and in the highest kingdom of the most High is their dwelling: as of light and shade the knowers of the Brahman speak of them, and those of the five fires and those who kindle thrice the fire of Nachiketas. May we have strength to kindle Agni Nachiketas, for he is the bridge of those who do sacrifice and he is Brahman supreme and imperishable, and the far shore of security to those who would cross this ocean.’ ”
There is little doubt that these verses speak to esoteric concepts that have their origin in the Veda, and that required considerable background and orientation for the seeker to get to the true sense being communicated. We can however hopefully find the right direction for understanding this material. The two that are basically joined in the world of manifestation are referenced elsewhere as two birds on a common tree, and represent the individual Atman, or soul, and the universal Divine, or the Kshara and Akshara Purushas, the manifest and the unmanifest. The light and shade are references to what other Upanishads may call the higher and the lower knowledge, which provide knowledge of the Brahman as well as knowledge of the outer world of manifestation.
The five fires is a reference to what is called the “Pancha Agni” sacrifice, which commentators for the most part identify as being about creation, focused on the earth, mid-world (vital world) and the heaven, bhur, bhuva, swar, matter, life and mind, as well as man and woman. Sacrifice has not only an outer sense of a ritual in the vedic tradition, but as Sri Aurobindo has shown, carries an esoteric meaning of the inner aspiration, the fire of concentrated energy, tapasya.
The three fires of Nachiketas are generally recognized to represent the aspiration or concentrated energy at the level of the physical, vital and mental planes of activity. To achieve the transition to the divine standpoint from the human egoistic standpoint, all aspects of our human existence need to be taken up and need to respond to the change. Through the integral aspiration at all levels of the being, the transition from the ego to the divine viewpoint takes place, and at that point, the individual and the Supreme are unified.
There are alternative readings to these verses due to the ancient language and references, but the general sense seems to prevail across the different specific interpretations.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129