At its core, a zero-knowledge proof is a cryptographic technique that lets one party, called the prover, convince another party, called the verifier, that a particular statement is true, without the verifier needing to see or trust any of the underlying data that makes the statement true in the first place. Applied to Optimism's proposed Bitcoin verification system, the statement being proven is specific and narrow: that a particular Bitcoin transaction is genuinely included in a particular Bitcoin block, as reflected in that block's Merkle tree, the cryptographic structure Bitcoin uses to compactly represent every transaction within it.