What is the definition of a 'Sadeh Migrash':
Targum Onkelos and Targum Yonasan: It is the open area 1 surrounding the town.
Measuring three thousand Amah Amos.
What is the Torah adding here that is not included in the previous Pasuk?
Targum Onkelos and Targum Yonasan: It is incorporating the open area surrounding the towms of the Levi'im in the Din of Batei Arei ha'Levi'im.
Rashi #1: The Torah is teaching us here, that, if the treasurer of Hekdesh sells a field that a Levi declared Hekdesh, it reverts to the Levi in the Yovel. 1
Rashi #2 (in Rosh Hashanah, 29a): Even a house in a walled town, which does not revert to a Yisrael in the Yovel, reverts to a Levi.
Erchin, 33a: It is prohibitting the conversion of field or a town of the Lei'im into a Migrash or vice-versa. 2