The Super Star is the power source of the Mushroom Kingdom, and a plot device in Super Mario Bros. and the Secret of the Super Star. It is a Power Star with a smiley face that provides an insane amount of energy to entire cities and can also grant wishes.
Prior to the events in this game, Blue Toad and Yellow Toad spy on Bowser, discovering that he is attempting to steal the Super Star in order to make his wish of falling in love with Peach come true. Once the two Toads tell the news to Mario and Luigi, they set out to stop Bowser. Whilst they are away, however, Bowser sneaks into Peach's Castle and steals both the Super Star and Princess Peach. In the game's ending, Mario, Luigi and the Toads rescue both Peach and the Super Star, which they return to the Mushroom Kingdom.