A Red Coin is a special kind of coin from the Mario series. Usually, the point of red coins is to try to collect all of them in certain levels.
The red coin's first appearance was in the game Super Mario World 2: Yoshi's Island. But their first 3D appearance was in Super Mario 64 where collecting eight of them got Mario a Power Star. In almost every game they've appeared in, Mario had to collect eight. For example, in Super Mario Sunshine, there would be levels in which you collect eight red coins to get a Shine Sprite. They have a minor role in the New Super Mario Bros. games. The player has to touch a red ring, after that eight red coins would appear. If he or she collected all eight coins in the time limit, he or she would get an item (a power-up or a 1-up). The red coins are absent in Super Mario Galaxy. Instead there are Purple Coins. The player has to collect 100 of them to get a Power Star.