Optimism ($OP) is an Layer-2 scaling solution of Ethereum using optimistic rollup.Optimism benefits from the security of the Ethereum mainnet and helps scale the Ethereum ecosystem by using optimistic rollup. That means transactions are trustlessly recorded on Optimism but ultimately secured on Ethereum.
Optimism is an "Optimistic Rollup," which is basically just a fancy way of describing a blockchain that piggy-backs off of the security of another "parent" blockchain. Specifically, Optimistic Rollups take advantage of the consensus mechanism (like PoW or PoS) of their parent chain instead of providing their own. In Optimism's case this parent blockchain is Ethereum.