It is a way to create reasonably isolated environment without the VM overhead. On Linux it is usually called container and implemented in terms of clone() and cgroups, BSDs have jails, Solaris has zones and for Plan 9 that is trivial concept. What is special about the Linux and Plan 9 cases is that the implementation is based around stuff that is generic and not strictly tied to this use case.

As a side note: I’m somewhat bewildered by the docker-controlled-by-kubelet-on-systemd architectures as there is huge overlap in what these layers do and how they are implemented.