This is a popular question on this subreddit so feel free to do a search. The answer is that Russia expanded quickly into the relatively sparsely populated areas on northern Europe and Siberia that weren't claimed by any other Great Power states. The expansion was quite similar to Western European colonial expansion in the Americas, except the Russians didn't have to cross an ocean to do it. The primary motivation were economic/commercial benefits of the pelt trade and (later) land routes to East Asia. The Russians faced some resistance from the local peoples but the Russians were equipped with better gunpowder weapons, among other things. After the collapse of the Golden Horde (itself a remnant of the Mongol Empire) in the 15th century, there was no power in northern Asia that could match the Russians or wanted to claim that land, so Russians simply expanded without too much difficulty.