Aside from drawing lines around the city that will be the basis for the 2,000 ama boundary for tehum, the Mishna on our daf presents the position of Rabbi Meir who believes that a karpaf (an additional enclosure outside the city) of about 70 amot should be drawn around the city, and measuring the 2,000 amot should begin from there. The Hakhamim disagree, ruling that such a karpaf would only play a role if two cities are close enough that an additional area such as that could connect them, making them one city.
Rava in the Gemara explains that the source for the idea of adding an area outside the city that is considered part of the city for the purposes of tehum is the passage (Bamidbar 35:4-5) that discusses how the cities of the Levites were to be set up upon entering the Land of Israel. The passage instructs “from the wall of the city and outwards…and you should measure outside the city, to the east 2,000 amot…”
From this we see that before the measurements begin, an area is added “outwards” beyond the walls of the city.
Although the amount that is to be added does not appear in the Torah, the Ra’avad explains that the Gemara assumes that we will add the area of a normal courtyard – that is, the hatzer (courtyard) of the Mishkan.
The Ritva understands that the Hakhamim in the Mishna reject Rabbi Meir’s interpretation of the passage entirely. They do not believe that anything can be learned from the Levite cities for our purposes. When they allow two cities that are nearby one-another to be considered joined for the purpose of tehum, it is based simply on the closeness of the cities, not on the passage in.
The Jerusalem Talmud, however, understands the disagreement between Rabbi Meir and the Hakhamim to be based on that biblical passage. According to Rabbi Meir, we learn from it that every city has an invisible addition around it. In arguing with him, the Hakhamim read the passage “from the wall of the city and outwards…” to mean that we measure from the wall of the city in normal cases. In a case where two cities are neighboring one-another, then we look “outwards” as well, by adding the additional karpafs to them.