Artscroll translates them as “Chaldeans”, I believe. However, that doesn’t match up with the history. As far as I could find (I once researched this because I was curious when I learned the Gemara in Shabbos), there was no real major upheaval in Babylonia during the Tannaitic or Amoraic eras. I am no expert, but I would guess that it refers to small pagan Assyrian tribes that probably took over small locales that had most of the Jewish population and these pagan tribes would not have been sympathetic towards Judaism in the slightest.