What do they mean? I don't find an answer on the website. Forum help and information seems to be pretty skimpy.

Anybody can assign 1-5 stars to a thread based on their perceived value of it.

I guess that's why you don't have any answers here, and one star. :-)
There's another forum for asking questions about the site itself.

Thanks for the answer. Now I wonder if I unintentionally added stars when I responded to some threads!

You must mean the Feedback/Suggestions forum. I guess I should have looked at it closer, but the subtitle doesn't make it obvious that's  the place to ask questions about the forums.