The Golden Horde was one of the four Khanates that the greater Mongolian Empire broke up into after the death of Genghis Khan.

From wikipedia:

The Golden Horde (Mongolian: Алтан Ордын улс Altan Ordyn Uls; Tatar: Altın Urda; Russian: Золотая Орда, Zolotaya Orda; Turkish: Altın Ordu) is a Russian designation for the Mongol[1][2][3] — later Turkicized[4] — khanate established in the western part of the Mongol Empire after the Mongol invasion of Rus in the 1240s: present-day Russia, Ukraine, Moldova, Kazakhstan, and the Caucasus. At its peak the Golden Horde's territory included most of Eastern Europe from the Urals to the right banks of the Dniper River, extending east deep into Siberia. On the south the Horde's lands bordered on the Black Sea, the Caucasus Mountains, and the territories of the Mongol dynasty known as the Il-Khans.

The Golden Horde was mentioned numerous times throughout the book and loosely referenced even more. Bold, the main character of book 1, was a soldier in Temur's (Tamurlane) army who was staging a campaign against the west and had already fought battles against the Golden Horde. On page 27 Bold comments on how he had killed quite a few members of the Golden Horde, a loose reference to the battles fought between his former empire and that one. Also, the Golden Horde is mentioned more than once as a reference point to where Bold thinks he is in the world like on page 18. Robinson is using this known historical reference to keep his story in perspective and within a correct date/time period.