Certificate Transparency is a public logging system for TLS certificates. Certificate authorities are expected to publish newly issued certificates to append-only logs, where browsers, domain owners, and security monitors can inspect them. The logs are usually organized with Merkle tree structures so changes can be audited efficiently and tampering is easier to detect.
It matters because certificate mis-issuance can let an attacker create a valid-looking certificate for a domain they do not control, which can support phishing, traffic interception, or impersonation. Defenders use Certificate Transparency to spot unexpected certificates for their domains, investigate suspicious issuance, and respond quickly with revocation or incident handling. In modern web PKI, it is a key backstop that makes certificate issuance more observable and harder to abuse silently.