Fedora Silverblue is a variant of Fedora, which became an official variant on October 30, 2018 with Fedora 29's release. Silverblue provides an immutable filesystem, along with rpm-ostree, which provides fully atomic upgrades, easy rollbacks, and a familiar workflow to OSTree-based servers.
Flatpaks are used on Silverblue. They are a way to make apps that work on all distros, like AppImages and Snaps. Unlike rpm-ostree packages, they don't require a restart. Flatpaks however are sandboxed apps, which provides security, but also a little less ability to use items outside the sandbox, which could be troublesome for some users.
rpm-ostree is the package manager for Silverblue. It supports package layering, which allows for RPMs to be installed for "OS extensions" like fonts, utilities, and printer support. However, once you install a package, as the filesystem is immutable, you must reboot into a new image created to use the package. This doesn't occur with Flatpaks.