This command is part of the Flipcoin minigame. Viewers can execute the command, specifying an amount of coins behind it and get a 50% chance of doubling their input - or lose all. You can also specify your own flipcoin command in Configuration → Gambling.
!flipcoin amountReplace "amount" with the amount of coins you want to gamble.
The Coins System must be enabled, minigame Flipcoin must be activated.
This command can be used by every viewer.