Certificate Transparency (CT) is a protocol designed to fix several structural flaws in the SSL/TLS certificate ecosystem. Described in RFC 6962, it provides a public, append-only data structure that can log certificates that are issued by certificate authorities (CAs).
By logging certificates, it becomes possible for the public to see what certificates have been issued by a given CA. This allows site operators to detect when a certificate has been issued for their domains, allowing them to check for unauthorized issuance. It also allows browsers and root stores, and the broader community, to examine the certificates a CA has issued and ensure that the CA is complying with their expected or disclosed practices.
For more information about how Certificate Transparency works and its role in supporting the web PKI, you can find a helpful introduction to CT at https://certificate.transparency.dev.
Chrome requires all publicly-trusted TLS certificates issued after April 30, 2018 to support CT in order to be recognized as valid. This site provides details on what is required. Any questions should be directed to the email@example.com list.