In case the way I word it helps, containers are a very similar concept to FreeBSD jails or Solaris Zones - so once you get what a container is you may appreciat

In case the way I word it helps, containers are a very similar concept to FreeBSD jails or Solaris Zones - so once you get what a container is you may appreciate some of the power that already existed in those systems. It's essentially a partition of the system: picture a kernel, but an additional instance of the network stack for each container, a certain mount-point that becomes the "root" file system for all actions inside that container, etc. In the end you get very similar functionality to a VM, but because it's just partitions of a kernel it's much more integrated than some third-party hypervisor. This lets you do things like inspect processes that are inside your containers as though they were native processes, etc...