Gas in Ethereum is a unit of measurement used to determine the amount of computational effort required to execute a transaction or smart contract on the Ethereum blockchain. Gas is paid in Ether (ETH) and is used to incentivize miners to include transactions in their blocks.
For example, if you wanted to transfer some Ether from your wallet to another wallet, you would need to pay a certain amount of gas for the transaction to go through. This amount of gas is determined by the complexity of the transaction. The more complex the transaction, the higher the amount of gas you need to pay.