At a high-level, Optimistic Rollup is a Layer 2 scaling solution that is generalizable to any type of Ethereum transaction. It drastically reduces gas costs per transaction, while preserving the decentralization and security properties of the Ethereum main chain. On Optimistic Rollup (OR), you can send ETH and interact with any smart contract, all with much cheaper fees.
Optimistic Rollup is a much simpler mechanism than Plasma or State Channels, solving the infamous dataavailability problem by directly posting all critical data on-chain as calldata. This means we submit every single OR transaction on-chain. Now, this might sound expensive, but we actually still achieve substantial gas savings because none of those transactions are actually executed on-chain.