Star Coins are helpful coins that are found in game that you can get, However, they disappear after a limited time.
Yellow Star Coins: The Yellow Star Coins are only worth 1 coin and gives you 1 health. They can be found mostly anywhere and after you kill some enemies.
Red Star Coins: The Red Star Coins are only worth 2 coins and gives you 2 health. They can be found in some places, except for ! Blocks, Black Bricks, and Wooden Crates. You also need to find 8 of them in painting worlds to get a Power Star, and are found mostly anywhere (even when you battle Bowser or are trying to unlock a character)
Blue Star Coins: The Blue Star Coins are only worth 5 coins and gives you 5 health. They can be found by Koopas and Blue Star Switches (but when you use them, you have to get them quick, or they will be gone) some Boo's, and if you run around one of the eyeballs, they will be defeated and drop one.