Gwei is a denomination of Ether (ETH) used to measure gas prices on the Ethereum network. One gwei equals 0.000000001 ETH (one billionth of an ETH). Gas prices are typically quoted in gwei to make the small amounts more readable.
Example
“The trader estimated their Ethereum transaction cost at 50 gwei per unit of gas, meaning a standard transfer requiring 21,000 gas would cost approximately 0.00105 ETH in fees.”