The Mishna (92a) closed with the ruling that a woman who was told that her husband had died, and accepted betrothal from another man – but never consummated the marriage – will not be considered a divorcee (i.e. she will still be permitted to a kohen), even if her second husband gave her a get (a writ of divorce). Rabbi Elazar ben Matya derives this from the passage (Vayikra 21:7) that a woman who has received a divorce “from her husband” is forbidden to a kohen. In our case, however, the second man was never truly her husband – as she was already married – so the divorce has no significance whatsoever.
In our Gemara, Rav Yehuda quoting Rav critiques Rabbi Elazar ben Matya’s teaching. He says that Rabbi Elazar ben Matya “should have taught this verse as a pearl but in fact he taught it as earthenware shard (i.e. he could have arrived at a more significant conclusion).” Rather than learning from that passage that a meaningless get has no significance, he could have derived that even someone who is only divorced “from her husband” cannot marry a kohen. The intention here is a rule called rei’ah ha-get, “the trace of a bill of divorce” where a woman who receives an invalid divorce – e.g. when her husband writes a get that states that she is divorced from him but cannot marry anyone else – will still be forbidden to a kohen.
In his commentary to the Ri”f, the Ri Almandri writes that Rabbi Elazar ben Matya’s original teaching is referred to as an “earthenware shard” because it is unnecessary, as it is obvious that someone who is not married cannot give a meaningful divorce. The Arukh LaNer takes another approach, pointing out that according to the Rambam the concept of rei’ah ha-get that prohibits a woman from marrying a kohen is Rabbinic in origin. Apparently the Rambam understands that by using this passage to teach the rule of the Mishna, Rabbi Elazar ben Matya shows that he rejects the possibility that rei’ah ha-get can be derived from it as a Biblical prohibition.