The DigitalOcean Container Registry (DOCR) is a private Docker image registry that allows users to store and manage private container images. To clean up unused data, DOCR provides on-demand garbage collection, which puts the user's registry in read-only mode while allowing them to pull images during the process. The garbage collection works in two phases: scanning the registry for entities and updating metadata, followed by actual deletion of garbage. This process helps reclaim storage space, simplify developer workflows, and promote better resource management, ultimately maintaining cost-effective and efficient containerized infrastructure.