Gas is the unit of payment for commission in Ethereum.For example, a transfer from wallet to wallet costs 21000 gas. The price of gas ingwei - ether-penny. Gwei = 0.000000001 ETH.If everything is clear with the transfer price, then the cost of recording or executing a smart contract depends on its complexity - the more operations, the more gas is required for its execution. Check the website that about Ethereum Gas Tracker