The gemara brings a braita relating to the difference in halacha between a tube created and attached to the ground or attached to the ground first and then hollowed out while attached to the ground. If the former case, it is considered a vessel and therefore water can’t be passed through there to fill up a mikveh as it is considered mayim sheuvim. In the latter case, it is considered attached to the ground and is not a vessel adn therefore one can fill up a mikveh through there. The gemara asks a question – this doesn’t seem to match Rabbi Eliezer or the rabbis opinions. First the gemara needs to establish which argument between Rabbi Eliezer and the rabbis is the question referring to and then it answers which opinion in that argument matches the braita.