A superstar is someone who's either very famous or extremely good at what they do. Legendary baseball player Babe Ruth was the first person called a superstar in print, around 1920.
You might think of a superstar as a popular musician, like Taylor Swift, or maybe a famous actor or well-known athlete. It's true, Jackie Chan and LeBron James and Serena Williams all qualify as superstars, but if you're really good at what you do, you're a superstar too. If your brownies are the most delicious your family has ever tasted, you're a superstar baker.