Learn in the textbookApplying Encryption and Digital Signatures
DefinitionA verification key (検証鍵) is the key used to check whether a received digital signature is valid: the signer's public key (公開鍵).
ExplanationBecause the verification key can be made public, anyone who obtains the signer's verification key can check the signature. Whether that public key really belongs to that person must be confirmed separately, which is why a trusted certificate authority (認証局) issues an electronic certificate (電子証明書) that ties the public key to its owner.
AnalogyIt is like a sample imprint of your seal given to others in advance, used to check whether the stamp on an arriving document is genuine.