Optimistic Rollups & Fraud Proofs

Optimistic rollups are a layer-2 scaling solution for blockchains that bundle multiple transactions together and post them to the main chain as a single transaction. The "optimistic" part means that these bundled transactions are assumed to be valid unless proven otherwise. Fraud proofs are the mechanism by which someone can challenge the validity of a rollup transaction. If a fraud proof is successful, the invalid transaction is reverted, and the submitter of the fraudulent transaction is penalized.