Certificate Transparency (CT) is a system of public, append-only logs that record SSL/TLS certificates as certificate authorities issue them. It was designed to make it harder for certificates to be issued secretly or by mistake, and anyone can search the logs.
For enforcement work, CT logs can be useful because sites that move to a new domain usually need a new certificate, and that certificate shows up in the logs. Searching for familiar naming patterns can surface a relaunched site early, so takedowns can continue against the new domain.
CT logs are one of several signals that can help with tracking sites that change domains. They show that a certificate was issued, not who operates a site, so they're a lead to investigate rather than proof.