Gwei is a denomination of ETH. This can be compared to sats for Bitcoin or the cent for the Euro. 1 Gwei is equivalent to 0.000000001 ETH.
Gwei is often used to express the gas-fee on the Ethereum network. The gas-fee is a small price paid to perform a transaction on the Ethereum network. These are often small amounts of ETH making it easier to express in Gwei than in Ether. For example, a gas-fee involves amounts of 0.00000002 ETH. In such a case, it is easier to express this price as 20 Gwei.
