Gas literally means: gasoline, a kind of fuel. When you send tokens, execute contracts, transfer coins, or do other things on the ether block, the computer needs to perform calculations, and this calculation process requires network resources. You have to pay a "fuel fee" (that is, Gas) to let the computer work for you and let the miners process the transactions for you. Technically speaking, Gas exists in the Ethereum virtual machine as a counting unit of computational workload. Gas fees are fully paid in ETH.