Optimism, formerly the Plasma Group, is the development team behind Optimistic Rollup's extension technology. Rollup is a smart contract on Ethereum that connects the Ethereum main chain to L2. Rollup receives transaction data from the Ethereum main link, sends it to L2, which performs the calculation, and then receives the result of L2's calculation. Note that Rollup comes in two variants: Optimistic Rollup and ZK-Rollups. Optimism only uses Optimistic Rollups.