Czar is a Russian word for ruler or emperor. Those kinds of czars are long gone, but we still use the word to describe people in charge of something important.
Up until the early 20th century, the ruler in Russia was a man called a czar who had total power, like an emperor or dictator. In English, czar has hung around as a word for anyone in charge of anything. If you run a book club, you might jokingly call yourself a book czar. In the U.S. government, people in charge of large departments are often called czars — like the housing czar or labor czar.