ZenML is and always will be open-source at its heart. The core framework is freely available on Github
and you can run and manage it in-house without using the cloud product. On the other hand, the cloud product offers one of the best experiences to use ZenML, and includes a managed version of the OSS product, including some cloud-only features that create the best collaborative experience for many companies that are scaling their ML efforts.
You can see a more detailed comparison here