When Nachiketa had gone into the Yamaloka at that time he got the self-enlightenment.
How Nachiketa got the Enlightenment by the lord Yamaraj?
Here, I got some point about Self-enlightenment. But, I want to know some detail information about self-enlightenment.
https://en.wikipedia.org/wiki/Nachiketa
For his third boon, Nachiketa wanted to learn the mystery of what comes after death.
Yama was reluctant on this question. He said that this had been a mystery even to the gods. He asked Nachiketa to ask for some other boon, and offered many material gains.
But Nachiketa replied that material things will last only till tomorrow. He who has encountered Death personally, how can he desire wealth? No other boon would do. Yama was secretly pleased with this disciple, and elaborated on the nature of the true Self, which persists beyond death. The key of the realization is that this Self is inseparable from Brahman, the supreme spirit, the vital force in the universe. Yama's explanation is a succinct explication of Hindu metaphysics, and focuses on the following points:
The sound Om! is the syllabus of the supreme Brahman
The Atma, whose symbol is Om is the same as the omnipresent Brahman. Smaller than the smallest and larger than the largest, the Soul is formless and all-pervading. The goal of the wise is to know this Atma.
The Atma is like a rider; the horses are the senses, which he guides through the maze of desires.
After death, it is the Atma that remains; the Atman is immortal.
Mere reading of the scriptures or intellectual learning cannot realize Atma.
One must discriminate the Atma from the body, which is the seat of desire. Inability to realize Brahman results in one being enmeshed in the cycle of rebirths. Understanding the Self leads to moksha
Thus having learned the wisdom of the Brahman from Yama, Nachiketa was freed from the cycle of births.
Go through Yudhisthira’s definition of self-realization.
Yudhisthira on Emancipation
There are two well-known paths (for us), viz, the path of the Pitris and the path of the gods. They that perform sacrifices go by the Pitri-path, while they that are for salvation, go by the god-path. By penances, by Brahmacharya, by study (of the Vedas), the great Rishis, casting off their bodies, proceed to regions that are above the power of death. Worldly enjoyments have been styled as bonds. They have also been called action. Liberated from these two sins (viz, bonds and action) one attains to the highest end. ….As a person on the hill-top looketh down upon men on the plain below, so he that has got up on the top of the mansion of knowledge, seeth people grieving for things that do not call for grief. He, however, that is of foolish understanding, does not see this. He who, casting his eyes on visible things, really seeth them, is said to have eyes and understanding. The faculty called understanding is so called because of the knowledge and comprehension it gives of unknown and incomprehensible things. He who is acquainted with the words of persons that are learned, that are of cleansed souls, and that have attained to a state of Brahma, succeeds in obtaining great honours. When one seeth creatures of infinite diversity to be all one and the same to be but diversified emanations from the same essence, one is then said to have attained Brahma. Those who reach this high state of culture attain to that supreme and blissful end, and not they who are without knowledge, or they who are of little and narrow souls, or they who are bereft of understanding, or they who are without penances. Indeed, everything rests on the (cultivated) understanding!’
Mahabharata Santi Parva Section XVII
You may also go through the passage posted below.
When men of knowledge, conversant with the rules of Yoga, become as fixed as a stake of wood, and as immovable as a mountain, then are they said to be in Yoga. When one does not hear, and smell, and taste, and see; when one is not conscious of any touch; when one’s mind becomes perfectly free from every purpose; when one is not conscious of anything, when one cherishes no thought; when one becomes like a piece of wood, then is one called by the wise to be in perfect Yoga. At such a time one shines like a lamp that burns in a place where there is no wind; at such a time one becomes freed even from one’s subtle form, and perfectly united with Brahma. When one attains to such progress, one has no longer to ascend or to fall among intermediate beings. When persons like ourselves say that there has been a complete identification of the Knower, the Known, and Knowledge, then is the Yogin said to behold the Supreme Soul.
Mahabharata Santi Parva Section CCCVII