Assume it's valid, prove it if challenged

An optimistic rollup batches thousands of transactions together off-chain, then posts a compressed summary back to Ethereum. True to the name, it optimistically assumes every batch is valid rather than proving it upfront.

To keep this honest, there's a challenge window, typically about seven days, during which anyone watching can submit a fraud proof if they spot an invalid transaction in the batch. This is why withdrawing funds from an optimistic rollup back to Ethereum takes about a week: the network is waiting out that challenge period.