Ethereum is a huge network of computers linked together. As with Bitcoin, transactions are replicated to each of the computers on the network. To use the network functionality, the ETH token was first used as a kind of fuel.
But faced with the volatility of the ETH token, it was decided to create the gas system so that the network operating cost does not vary according to the price of the ETH token. The gas is simply used to assess the workload necessary to operate a smart contract on the Ethereum network.
The more complex the calculations for performing an operation, the more the gas price increases. You will pay less in gas if you make a simple ETH transaction than if you participate in an ICO for example.