With 1 license you are allowed to use Terragen on 1 machine at the same time.
Render nodes probably mean that you can render your scene on multiple machines at once.
Such a machine is called a node and that node is used to render, thus a render node.
I think it's a bit like this in a nutshell, I'm not 100% sure, so Oshyan or somebody else will probably chime in if there's more to it.
One thing is for sure: without additional render nodes, so when only having one TG3 license, you can still render your projects on your machine.