In the first Mishna in Massekhet Moed Katan (2a) we are taught that Hol HaMoed was a time during which a series of activities took place for the betterment of public facilities. These public works projects include a number of tasks that follow or prepare for the rainy season – for example, clearing the roads and mikva’ot and marking graves so that the people who are coming to bring sacrifices will not, inadvertently, become ritually defiled by contact with a grave and be unable to enter the Temple.
The Gemara on our daf asks for a source for the practice of marking graves, and brings a passage from the prophet Yehezkel (39:15) that describes the calamity of the war of Gog and Magog, and how it will take seven months for all of the dead to be properly buried so that the land of Israel will once again be tahor (ritually pure). The prophet describes the method that is to be used to carefully mark the graves, bone by bone.
This source for the halakha that graves must be marked (see Rambam, Hilkhot Tum’at Met 8:9) is introduced as a remez – an allusion – to the law, rather than as the actual source. Given the clarity of the story in Yehezkel, many of the commentaries ask why the passage is only considered a remez.
From Rashi it appears that since it is not presented as an obligation, but rather as a story, it cannot be considered a true source.
Tosafot suggest that the story can only be considered an allusion to the halakha because it is a description of an event that will take place “at the end of days.” Such a story cannot be the source for a present day halakhic obligation.
It should be noted that the Yerushalmi presents this as a true source text, not simply as a remez. In fact, it is not uncommon to find the Bavli discounting a source unless it appears in the hamisha humshei Torah (the Five Books of Moses), while the Yerushalmi accepts other sources from TaNaKH as well.