It's a bit deceptive but I'm pretty sure they mean that they ship a Ubuntu distro you can install on an unformated (no-OS) computer. Having said that the instructions I found assume you will find and install the OS yourself:
https://docs.docker.com/engine/installation/linux/ubuntu/
It could also simply mean that you don't need VirtualBox installed (bare metal normally refers to the difference between running as a guest OS on a VM and running on a physical box).