Put a private domain there and it'll show you data from your usage of 1.1.1.1.
On their page however, they claim "By contrast, 1.1.1.1 does not mine user data. Logs are kept for 24 hours for debugging purposes, then they are purged." under "What makes 1.1.1.1 more secure than other public DNS services?" [0]
[0]: https://www.cloudflare.com/learning/dns/what-is-1.1.1.1/