Each transaction on the Ethereum network requires computational resources to execute and therefore each transaction requires a fee. Gas refers to the fee required to successfully conduct a transaction on Ethereum.
This gas fee is charged and determined by the Ethereum network, not MetaMask.
Due to the high demand on the network to complete transactions, gas fees are high and are a problem that the Ethereum developers are working to scale.
You can learn more about gas fees and how transactions are processed on the Ethereum network here.