When the Pandavas were in exile, at that time, Narada muni approached their hideout and had discussion with Yudhishtira about Dharma.
Can someone briefly explain what that discussion was?
The discussion between Narada and Yudhistira occurs in the Tirtha-Yatra Parva a section of Vana Parva in Mahabharata.
Yudhistira asks about the merit attached to sacred pilgrimage and Narada recounts a conversation between Bhishma and Sage Pulastya.
"Bhishma said, '[...] But, O thou foremost of virtuous persons, if I have deserved thy favour, I will tell thee my doubts and it behoveth thee to dispel them, O holy one, I have some religious doubts in respect of tirthas. Speak of those to me in detail, I desire to hear thee. [...]"
"Pulastya said, 'O son, listen with attention. I will tell thee of the merit which attacheth to tirthas and which constituth the refuge of the Rishis.
What follows is a detailed account of all the sacred pilgrimage spots in Bharata Varsha. Bhishma visits these on Sage Pulastya's order and acquires merit.
"Narada continued, 'Having cheerfully spoken thus, the illustrious Rishi Pulastya, well-pleased, bidding Bhishma farewell, disappeared there and then. And Bhishma also, O tiger among men, well understanding the true import of the Shastras, wandered over the world at the command of Pulastya. Thus, O thou blessed one, did Bhishma end at Prayaga his highly meritorious journey to the tirthas capable of destroying all sins.
The narration concludes with Narada telling Yudhistira to meet Sage Lomasa and perform tirtha yatra with other ascetics to acquire the same merit.
Thou wilt, O son of Pritha, obtain merit consisting of the eight attributes, even like that which Bhishma, the foremost of the Kurus, had obtained of yore. And as thou wilt lead these ascetics to those tirthas, thy merit will be much greater.
With these, O mighty king, do thou meet by visiting these tirthas. And, O illustrious monarch, a great Rishi of immeasurable energy, Lomasa by name, will come to thee. Do thou follow him, and me, and by turns visit these tirthas, O thou virtuous one! By this, thou wilt acquire great fame.
Vaisampayana continued, "O great king, having comforted the monarch thus, the illustrious Rishi Narada, bidding farewell to the king, disappeared there and then. And the virtuous Yudhishthira, reflecting upon the subject, began to recite unto the ascetics the merit attaching to tirthas!"