Ethereum “gas” is analogous to the fees charged by miners for executing a transaction on the Ethereum Blockchain, but specifically for the execution of smart contract calls. While ETH is the currency of the Ethereum network, gas is a unit used to measure how much computation a certain task requires.
ETH and gas are not the same: the price of gas in ETH fluctuates and is mostly decided by a bidding process for priority of execution by miners (the price of gas is set in “gwei” - one gwei is one billionth of one ETH). If the network is congested with lots of transactions, the average gas price will rise, and so the same amount of computation on Ethereum will cost more ETH. If there is less demand on the Ethereum blockchain, the gas fees will decrease.
As of June 2021, there is an Ethereum proposal, EIP-1559, part of the “London” hard fork, that is proposing to revamp the entire gas model by removing the bidding process described above.