Immutable + atomic. Its similar to Android or IOS. It can be explained like that:

• big parts of the core OS cant be changed easily. Immutable means "you cant change/break it". This also applies to software, third party install scripts, viruses and other things that might break your system.
• atomic: updates are done like this:

1. The system is an image, as if you would live boot a usb stick
2. When you do an update, the package manager checks on the server for changes, I think it uses git. Only the changes are downloaded.
3. rpm-ostree has downloaded the diffs, updated packages basically. Instead of just replacing your local packages, from the full operating system on your machine it builds a new image. Remember, the image is like the live USB or CDROM you can boot and use but not change.
4. This new image is staged. This means if you reboot, you will boot into the updated version automatically. Updates go in the background and you will have a working system without any downtime. This is so much faster than for example Windows Updates or even standard Fedora "secure updates".
5. atomic means that if something in that process fails, you will simply not get an update. So updates cant break anything.
6. But dont forget its Linux and not Android. You can actually install what you want. This means during the "get updates phase" you can not only download "regular update packages" but also any other Fedora RPM package you want. This is called layering, as now this package is always added to your system on every update, as remember on every update your system would get resetted. You can also remove preinstalled packages, a common one on Fedora is the Firefox RPM.

That you can normally install apps is thanks to Flatpak, so you dont need to reboot on every install. The idea is to have a very slim core system and "outsource" as much as possible to Flatpak. This means at the same time, official packages, less work for the distro maintainers, and containerization.

In the future even more packages will be removed as native packages and installed through Flatpak. Buts still a developing technology and important things like native messaging or USB access (hardware security keys) are still missing.