On networks like Ethereum, "gas" is the unit used to measure the computational work a transaction requires, and the gas fee is what you pay — in the network's native currency — to get that transaction included in a block. More complex transactions (like interacting with a smart contract) typically cost more gas than a simple transfer.
Gas fees rise and fall with network demand: when lots of people are transacting at once, fees go up, because block space is limited and users are effectively bidding for inclusion. This is one reason scaling solutions like Layer 2 networks exist.