A rollup means instead of putting transactions directly on Ethereum they go to a Coinbase-managed chain that later commits those transactions and their results

A rollup means instead of putting transactions directly on Ethereum they go to a Coinbase-managed chain that later commits those transactions and their results to the Ethereum chain in a cheaper way than executing them on-chain directly. "Optimistic" means there is a period where people can prove to the chain that Coinbase lied about the transaction results and after that period it's assumed they're correct.