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Investopedia: Ethereum gas fee

Gas is the fee required to successfully conduct a transaction or execute a contract on the Ethereum blockchain platform. Fees are priced in tiny fractions of the cryptocurrency ether (ETH) - denominations called gwei (0.000000001 ETH). Gas is used to pay validators (former miners) for the resources needed to conduct transactions.

The exact price of the gas is determined by supply, demand, and network capacity at the time of the transaction.

Many transactions also include tips, which are added to the gas price (the more you pay, the faster your transaction is completed). The lower a user estimates their gas limit, the lower the priority in the queue they will be.

A transaction fee is similar to the fee you pay for a money wire transfer. You're paying the service provider for using their network.

Another factor to consider is that supply and demand for transactions dictate gas prices—if the network is congested, gas prices might be high. On the other hand, they could be low if there is not much traffic.

Why Do I Have to Pay a Gas Fee?

The Ethereum gas fee exists to pay network validators for their work securing the blockchain and network. Without the fees, there would be few reasons to stake ETH and become a validator. The network would be at risk without validators and the work they do.