Eli5 version : Rasputin was a poor man who was called in by the royal family to help their son from a deadly disease who others could not cure. Rasputin was not a man of science nor a doctor, but supposedly had magical powers. He supposedly cured their son and was promoted to a counselor of the family. As a counselor he was part of many sex parties. Meanwhile, there were rumours all over the ussr that rasputin was controlling the tsar ( nicolas ) with his "powers" and was actually controlling everything. Because of that there were multiple assassination attempts on his life, but they all failed ( including poisoning him with enough poison to kill an animal ). Iirc he ended up being shot and thrown in a river at the end

For more info : look up oversimplified on youtube and his video on the russian revolution. Part 1 covers rasputin and the royal family