WTF ARE FRAUD PROOFS?

Summary from the TLDR Crypto newsletter

Optimism's introduction of fraud proofs means that bridged assets can be withdrawn without Optimism's approval and anyone can review and challenge OP Mainnet transactions. Previously, the Ethereum settlement layer had to trust Optimism's transaction history without proving it. Looking forward, other OP Stack chains like Base and Mantle should be able to implement fraud proofs and become Stage 1 L2s much quicker.