Optimism is a Layer 2 Ethereum blockchain meant to help with scaling. Optimism realizes the network efficiency problem that Ethereum faces. That is, in its early days, the low throughput that the Ethereum network presented didn’t stand in the way of its ability to handle its traffic. But as time has gone on, as the industry has matured, and as Ethereum has become the largest smart contract platform in the world, its low efficiency has really begun to show at scale.
Optimism is built to increase the throughput of the network without actually changing anything about how it operates. It processes transactions off chain and works with Ethereum’s existing consensus mechanism.