‘Gas’ or "Ethereum transaction costs" is the cost of performing a transaction or contract on the Ethereum network. Every operation that is performed by a transaction or contract on the Ethereum network requires a certain amount of gas. Operations that require more computational resources cost more gas than operations that require fewer computational resources.
Gas or "Ethereum transaction costs" is important because it helps ensure that an appropriate fee is paid when operations are submitted to the network. By requiring a cost for each operation, the network is less likely to become bogged down by performing intensive computational work that isn't valuable to anyone.
Gas costs are charged in Ether (ETH), which is the native token of the Ethereum network and the token with which miners are rewarded for producing blocks. Gas prices are usually denominated in Gwei. A Gwei is a fraction of an Ether.
The cost of each transaction is decided by the network’s miners, who can refuse to process a transaction if the gas cost is too low. To pay gas / transaction costs, you simply need to add ETH to your wallet.