Which means you're not allowed to flatten out, smooth out, or even out the field in order to prepare it for plowing. But if you're flattening out the field in order to thresh—because, for example, it seems they used to thresh on a flat surface—you’re flattening out the whole area for the purpose not of plowing, but of threshing. It would be permissible, because I guess you could thresh on Chol HaMoed (intermediate days of the festival) for the purpose of the Moed (festival).
So the Mishnah Berurah says, which means you need more flat ground for the threshing process. So I guess when you flatten or even out a large area of land, it's noticeable that you're doing it for threshing. And some say the exact opposite: for threshing, you don't need so much flattening; you need more flat land—you need more flat land for plowing. We learned the Machloket Rishonim (dispute among early authorities) when we learned the Gemara: What is the flat land? Is it for the plowing or for the threshing? They'll say it. They'll say it either way.