A gas fee is the payment required to have a transaction processed and recorded on a blockchain such as Ethereum. "Gas" measures the computational work a transaction demands; the fee is that work priced in the network's native coin.
Fees are set by supply and demand for block space, not by any fixed price list. When the network is busy, users effectively bid against each other and fees spike; when it is quiet, they fall. A simple transfer costs less gas than a complex smart-contract interaction.
Gas fees can make small transactions uneconomic at busy times, since a few dollars of activity might carry a fee of similar size. This is one reason newer networks and layer-2 systems compete largely on how cheaply they can process transactions.
Worked example
Sending tokens on Ethereum during a busy period can cost more in gas than the same transfer would at a quiet hour.
This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.