Are you asking what the output of that command means? I’m looking at the rbenv documentation, and I think it indicates that rbenv only knows of one version of ruby – the version that came installed with your system.
Edit: If that’s not the answer you were looking for, can you tell us what you expected to see? Did you have other versions installed on that machine?
Edit 2: If you want to see a list of versions you can install onto your machine, but aren’t installed yet, run:
rbenv install --list
Here is more documentation for this command. This will help!