TLDR: It's an old initiative to make using systemd-nspawn much easier, including a hub of images compiled with mkosi and signed using Sigstore.It was put on hold about two years ago, but we've revived it. The hub is available at https://hub.nspawn.org/home

The current CLI implementation matches most of the Docker's used features, such as search/pull/exec/run/network/cp/events/stats/build, respecting the entrypoints, propagating labels, ports and resource limits management, and more. Support for other registries such as DockerHub and any OCI registry has been added as well.

Documentation can be found in: https://nspawn.org/docs/

Edit: as an additional note, everything is exposed through a D-Bus API so that any other client/UI can make their own implementation.