What gas is
Ethereum runs programs, called smart contracts, on thousands of computers. To stop anyone from overloading the network and to pay those who run it, every operation is assigned a gas cost. ethereum.org defines gas as the unit that measures the computational effort required to execute specific operations. Sending ETH is cheap, while a complex contract interaction such as a swap uses much more gas.
Gas limit and gas used
When you send a transaction you set a gas limit, the maximum gas you are willing to let it consume. A standard ETH transfer requires 21,000 units. If a transaction needs more gas than its limit allows, it fails, and the gas spent up to that point is still consumed. Wallets estimate the limit for you, and unused gas above what was needed is returned.
Price per unit
The fee you pay is the gas used multiplied by the price per unit of gas. Prices are quoted in gwei, and each gwei is one-billionth of an ETH (0.000000001 ETH). Since the London upgrade, the price per unit has two parts: a base fee set by the protocol and an optional priority fee, or tip, paid to the validator.
What it means for you
When the network is busy, the base fee rises, and so does the cost of every transaction. Fees are paid in ETH even when you are moving another token, so you need a small ETH balance on any Ethereum address you want to use. Sending to a contract that does a lot of work, or failing a transaction, can still cost fees. Check the fee shown before you confirm, and wait for quieter periods if a transaction is not urgent.