Gwei is a small unit of ether - one billionth of an ETH - used to express gas prices. When a wallet shows a gas price of 12 gwei, each unit of gas your transaction uses costs 12 billionths of an ether.
Your fee is the gas the transaction needs multiplied by the gas price. A simple transfer uses little gas; a complex contract interaction uses far more. Prices rise and fall with demand for block space, so the same action can cost very different amounts at different hours. Glancing at the gas price before a transaction that is not urgent is one of the easiest ways to stop overpaying.
💬 Do you check gas prices before sending, or just press confirm?
Binance Academy covers this in more depth: https://www.binance.com/en/academy/glossary/gwei
Next lesson: EIP-1559: why part of every fee is burned
Trading crypto from Dubai since 2019.