Gas Fee
BasicsIn short The fee paid to a blockchain's validators for processing your transaction. Varies with network congestion — high demand means higher gas.
Block space is scarce, so transactions bid for it: the fee is an auction, not a price list. A simple transfer costs a fixed amount of work, while a complex contract call costs more, and both get multiplied by whatever the network is charging that minute. Two practical consequences: a failed transaction still pays, because the network did the work; and waiting for a quiet hour is often the cheapest optimisation available.