Gas fee
What it costs to have Ethereum's network perform a computation — the price is set by congestion, not by the transaction's value.
Every operation in the virtual machine has a fixed gas cost, and the total is multiplied by a price the user specifies. The construction exists to stop anyone submitting an infinite loop: running out of gas aborts execution, and the fee is charged anyway.
Because block space is limited, the price becomes an auction. During the 2021 NFT boom a simple transfer could cost tens of dollars, which in practice excluded small amounts. EIP-1559 introduced a burned base fee plus an optional tip in 2021, making prices more predictable. Layer 2 solutions exist largely to escape the cost entirely.