A gas fee is the cost of performing transactions or actions like minting/creating an NFT on the Ethereum blockchain
Other cryptocurrencies may simply call gas fees 'transaction fees', 'miner fees', or something similar. However, since Ethereum is currently the second-largest crypto by market cap, the term "gas" is often applied when referring to the fees involved in using other blockchains.
The amount of computational work your transaction requires and the current market price of gas. Gas fees are typically measured in a unit called "Gwei" (short for gigawei). The total cost is calculated by multiplying the gas price (in Gwei) with the amount of gas used.