Ethereum is the most frequently used blockchain. It requires “gas” to use—you can think of this as being similar to a processing fee for each transaction. You must have enough ETH in your wallet for the cost of your purchase and the gas. The gas fee enables your transaction to be processed on this network and to be stored on the blockchain. The gas fee per transaction can fluctuate based on network supply and demand. You can set a gas limit as the maximum amount of gas you are willing to use on any given transaction. If the actual amount of gas needed turns out to be lower than the limit you specified, the remaining gas will be returned to you. But if your limit is too low, either your transaction will not be processed or the transaction will fail and you may lose that gas. When making your purchase, your wallet can facilitate the calculation of the estimated gas price to make the transaction. See here for more information on how Metamask estimates gas fees.