The ‘gas fee’ is a transaction cost which you must pay to send tokens from one wallet to another, or to trade between two tokens.
On Ethereum the fee represents the network cost to process and validate your transaction in the next block. Units of gas are also represented in GWEI and the amount of GWEI required to complete one transaction will vary depending on congestion on the network.
In your MetaMask wallet you will be quoted a gas fee when you submit the transaction you would like to process.
You must then accept the gas fee to finalise the transaction. If successful it will be processed and you will receive a pop up confirmation.