Learn in the textbookThe Certificate Authority and the Operation and Revocation of Certificates
DefinitionA CRL (Certificate Revocation List) is a list the CA publishes of digital certificates that were invalidated (revoked) partway through, while still within their validity period.
ExplanationCertificates have a validity period and simply stop working once it ends. Separately, if the private key leaks or the owner changes organizations, the certificate must be cancelled even before that period is up. So a recipient checks not only that a certificate is within its validity period but also that it is not on the CRL.
AnalogyIt is like a list of blocked credit card numbers sent out to shops. Even if a card has not expired, a number on the list is refused.
Memory hookRevocation means taking back. Read it as a List of certificates taken back early, not ones that simply expired.