1. Russia always has harsh winters, in this case Napoleons army wa snot ready for such inconvenience. They froze, starved, and some eventually died.
2. Russia had a scorched earth policy, in which they burned crops, farms, and supplies. They abandoned cities, such as the capital, and left everything empty for the French. As the French arrived there was no one in the cities, but there was also nothing the French could use for their advantage. After some time, the French ran out of supplies and food, and to make it even worse, this was the time of the harsh winters. The French army was weak and was unable to fight back the Russian army which attacked the weakened army afterwards, and made them flee all the way back to France.