What was Ivan Ill, the Great, known for?
He was the Grand Prince of Moscow, on whose watch the Mongols retreated and the Russias united.
What we commonly think of as Russia began in 862 AD, but it was not a single kingdom or nation-state so much as a handful of rival principalities. And when Genghis Khan's Mongol horde (Minus the Khan; he died shortly before his troops reached Russia) conquered Russia in the 1200s, they weren't even that. The Mongols weren't exactly hands-on rulers, though; they didn't push Mongol culture on their conquests and after a while, their reign pretty much consisted of showing up once in a while to collect tribute.
Moscow, basically a fur-trading post at that point, was burned to the ground by Batu Khan (Genghis Khan's grandson)'s men. The Mongols, unable to collect tribute from smoldering embers, left and the survivors surveyed the damage. They realized that they could have avoided this fate pretty easily; Moscow was between an impenetrable forest and a wide river, with a narrow. strip of land between the two. They rebuilt, bigger and guarded the strip of land a lot more vigilantly.
As the Mongols splintered into less-powerful factions, the newly-fortified Moscow grew bolder about not paying the tribute. By the late 1400s, the Mongols quit asking. By this time, a Muscovite prince, Ivan III, began to exert his influence over the other Russian principalities and became the first Russian king to rule all of them. He was, in a practical sense, Russia's first Tsar (although the words "Russia" and "Tsar" were not yet commonly used).
He happened to be on the throne when the Golden Horde collapsed, and gets more credit than he deserves for kicking them out, but he did unite the Russias into a single kingdom, albeit one that got continually attacked from the west by Poland and Lithuania.