1st - 5th Verse
In the Second Chapter of Katha Upanishad Yama explains Nachiketa about death and real Self. Yama explains that there are two different things of contradicting nature i.e. the 'Sreyas' and as 'Preyas.' These chain man and try to arrest him with the worldly desires. The one who is able to follow 'Shreyas' attains good results here and the one who adopts 'Preyas' loses control of himself and of his spiritual progress.
The Sreyas and Preyas both come in search of a man. The wise man after analysing them is able to distinguish between them. He then deserts the path of 'Preyas' and adopts the path of 'Sreyas'. The unlearned takes the path of Preyas. Nachiketa could easily differentiate between them and could understand that the Preyas act as hindrances. He thus adopted the path of Shreyas. The path of ignorance and knowledge lead to different goals. Nachiketa was desirous of knowledge. There are people who live in the midst of ignorance and consider themselves wise.
6th - 10th Verse
The knowledge about the Heaven and Atman cannot be understood by a person who is not deeply interested in it. Many people think that it is the materialistic world that alone exists and thus, gets trapped into it. Yama tells Nachiketa that the Atma cannot be defined to a man of inferior knowledge. It can only be taught to a person who has identified himself with the Brahman. Then there would be no space for misunderstanding. Yama tells Nachiketa that the knowledge the latter has gained is not easily achievable.
11th - 15th Verse
The Brahman can only be understood by the person who meditates upon it truly. It is present within all beings and remains hidden within the heart. The knowledgeable person by continuous practice of meditation on the self understands the Brahman. A man who understands it and differentiates the root of dharma attains the smallest atom of the Atma. He then remains ever joyous. Nachiketa then asks Yama to explain that is beyond Dharma and Adharma, activity and cause. Yama tells him that it is the one that has been mentioned in the Vedas. Yama then explains the 'Om Mantra' (also written as AUM).
16th - 20th Verse
The syllable OM is the lower aspect of Brahman as well as the highest aspects of Brahman. One, who understands this, gets all his desires fulfilled. It is the best and the highest of all supports. One, who understands this support, becomes the blessed one in the Brahmaloka. A person who realises this gets rid of the cycle of birth and death. This Atma is not given birth to or produced by something. The Atma is birth less, all existent and immortal. A killer if thinks that he has killed the Atman is wrong. The Atma does not kill nor can it be killed. The soul, which is subtler than the subtle, greater the greatest, is present within all. One, who has completely renounced his desires, can see the Atma.
21st- 25th Verse
Sitting in a place the soul goes afar, sleeping it goes everywhere. Thus, who else can understand the Supreme Soul except Yama that resides everywhere in the forms of both pleasure and sorrow. A person who can understand the Atma residing in the bodies without a body does not get affected by any sorrow. It cannot be attained by anyone simply by learning the Vedas. Only by meditation upon the Atma rigorously can one truly understand the Atma.