Optimistic Rollups enhance blockchain scalability by conducting transactions off-chain while maintaining a connection to the main blockchain. In this model, every transaction is executed off the main chain but uploaded back to it in bulk. The 'optimistic' aspect lies in the presumption that all transactions are valid unless a challenge (proof of fraud) is submitted within a specific period. If a dispute occurs, the state of the rollup can revert back to the last valid state on the main chain, ensuring security while benefiting from lower costs and higher throughput.