Gas fees are payments made to cover the cost of the computing energy required to process transactions on the Ethereum blockchain. Here is a more technical explanation of gas fees.
The busier the network, the higher the gas fees will be to process transactions. This supply-and-demand mechanism enables miners to prioritize transactions that pay more, compelling other users to pay more to have their transactions processed more quickly and efficiently. Users of Ethereum can also choose to pay more for faster transactions.
You can see how busy the network is on Etherscan.