A coin is an item from Super Mario Bros . Coins can be obtained by hitting an empty Coinblock, a brick with the Manycoins entity, or simply picking one up in the world. In SE, they can additionally be obtained through Custom Enemies or Animations.
In Mari0: SE, coins are placed somewhat differently. They can be placed right on front of another tile, and to remove it, simply click the coin while having the coin tile selected, like a toggle switch. In 1.6 and below, coins were placed just like any other tile, so a mappack author would sometimes need to choose between a background or coins.
The primary use of Coins is that the player gains an extra life once they obtain one hundred coins. In SE, Animations can be set to only fire if the player has enough of them.