A champion is a winner, or someone who's really good at something. If you are a champion chess player, you are a superstar! When crowds sing Queen's "We Are the Champions" at football games, they are celebrating the fact that their team won.
Champion comes from the Latin word campionem for "gladiator, fighter." Rarr! No need to grab your sword, but a champion is also a person who fights for a cause. If you are the champion of fundraising, you keep pushing to raise money. As a verb, to champion means to protect or fight for something. You champion your little brother by defending him against meanies — no matter what, you are always on his side.