From the Super Mario Wiki
An Ace Coin.
Ace Coins are special coins that first appear in the Super Mario Advance remake of Super Mario Bros. 2. Ace Coins are large, red, and sport an "A," and are named "Ace Coins" because a player needs to collect all five of these coins in order to completely beat, or "ace," the level. If a player finds all five Ace Coins in this level, the player will obtain an Extra Life. However, Ace Coins are somewhat difficult to obtain, even risking the characters' lives for them. If the player plays through the level again, all collected Ace Coins will be transparent. Collecting them again still awards points, but they are not added to the running total. These coins are very similar to the Dragon Coins, and similar coins known as Advance Coins appear in World-e of Super Mario Advance 4: Super Mario Bros. 3, however, they're different colors.