Gwei is a very small amount of ethereum coin used to calculate transaction fees for sending ethereum to another. 1 Gwei = 0.000000001 Ether. It’s a denomination, just like 1 cent = 0.01 dollars. There are names for other denominations of Ether as well (see Ethereum Unit Converter), but Gwei is one of the most commonly used because gas prices are easily specified in Gwei.