Yama has told Nachiketas to ask for three boons.
One he has already asked; is that when he goes back to the earth, his father should treat him with affection.
Now the second boon.
स्वर्गे लोके न भयं किंचनास्ति न तत्र त्वं न जरया बिभेति। उभे तीर्त्वाऽशनायापिपासे शोकातिगो मोदते स्वर्गलोके।
I have heard that there is no fear in Swargaloka.
You are not there is Swargaloka; meaning Mrityu, death is not there.
There is no jara in Swargaloka, Nobody gets old in Swargaloka.
There is no hunger or thirst in Swargaloka.
There is no sorrow in Swargaloka.
In three worlds: Pitruloka, Yamaloka, and Manushyaloka you are taken over by these six; fear, death, old age, hunger, thirst, and sorrow.
These are completely absent in Swargaloka.
That is why people aspire for Swargaloka.
People work towards attaining Swargaloka.
As long as you are here on earth these six are there for you.
But then just like how a snake sheds its skin, through tapas and performing yajnas, you shed these six defects and attain Swargaloka.
Swargaloka is above the Chandra Mandala.
As you enter Swargaloka, whatever remains of these six, they come out of you.
स त्वमग्निँ स्वर्ग्यमध्येषि मृत्यो प्रब्रूहि त्वँ श्रद्दधानाय मह्यम्। स्वर्गलोका अमृतत्वं भजन्त एतद् द्वितीयेन वृणे वरेण।
I am sure that you would definitely know the nature of that Agni called Swagya Agni, the divine Agni which makes you attain Swargaloka.
It is by making use of Agni-vidya that Swargaloka is attained.
Nachiketa wants to know about this Agni, the nature of this Agni.
He says: I am shraddha dhana, I have shraddha, I am keen to listen, this is not a casual question, I am very keen to listen and understand this.
In the olden days, gurus would impart knowledge only to the most eager students.
Not like they have to finish the syllabus within a deadline and send them away.
They will keep on testing to see whether the student has developed an eagerness to attain knowledge.
In that also they would give only that much the student is capable of, deserves.
Nachiketas is saying that I am eligible to receive this knowledge from you because I am keen, I am eager to understand this.
As Chhandogya says:
यदेव श्रद्धया करोति तदेव वीर्यवत्तरं भवति।
Only that which is done with enthusiasm, eagerness, keenness, only that becomes powerful, result yielding.
श्रद्धावन्तोऽनसूयन्तो मुच्यन्ते तेऽपि कर्मभिः।
Keenness is essential for liberation from karma, says Gita also.
प्र ते ब्रवीमि तदु मे निबोध स्वर्ग्यमग्निं नचिकेतः प्रजानन् । अनन्तलोकाप्तिमथो प्रतिष्ठां विद्धि त्वमेतं निहितं गुहायाम् ॥ १४ ॥
Yamaraja said: yes, I will tell you about the Swargya-Agni.
The term प्रजानन् is important.
I will tell you because I know about it.
If you ask a noble person or a sage, in this case, a Devata, a doubt; if they have full knowledge then they will answer you.
Or else, they will say: I don't know.
To show that they are all-knowing, they will not tell you something that they are not sure about.
When you share knowledge with someone which you are not sure about, then you are weakening your atma.
So before you answer a doubt from someone always check; am I qualified to answer this question, do I have the right authentic answer?
Or else you will be harming yourself by answering that question.
I will tell you the secret of Agni-vidya.
Only those who know the secret of Agni-vidya can attain Swargaloka.
In fact, the foundation of the whole world is this Agni.
It is a secret.
I will tell you that secret.
Says Yamraja to Nachiketas, this is the second boon that Nachiketas wanted, knowledge about Swargya-Agni.