Every transaction on Ethereum costs ETH - we refer to this cost as the "gas price". It is paid to miners as an incentive to process transactions. Since the Etherum network has a limited amount of transactions per second, the prices per transaction can rise during periods of high network activity. Gas price is set within your wallet prior to making a swap. For interacting with the Uniswap app, you want to opt for a higher gas price to prevent swap expiration errors. In general make sure you review the gas fee in your wallet and select an average or fast transaction speed. If your transaction is too slow it will fail and you will lose the gas you paid to try to execute that transaction. Your gas fee cannot be refunded.
It's a good idea to double check the gas price suggested by your wallet on some other gas tracking app (eg. Gas Tracker).
You can learn more about concepts of gas price and gas limit here: