Atomic means the core OS packages are in an immutable container such that none of its individual components can be updated separately; instead the entire container is replaced with a newer version when the system is updated. This makes it much less likely for something to break during normal use, and easier to rollback updates if something does happen to break. The ideal use case is a containerized environment where each app you use is installed in its own container, like Docker, or is otherwise self-contained such as flatpak installers, and doesn’t rely on any of the system’s packages.