Hi Gumdrop,
In a nutshell, when someone is pinged, it means another poster is calling your attention to a particular thread that might be of interest to you. It will show up in the upper right screen in red in "new posts to you." In this case, it will serve to "ping" you to Minnesota topics that could be pertinent to you. Basically, it's just a call to attention to like-minded Freepers to Minnesota news you may have missed in the sidebar.
Sometimes people just ping themselves to a thread for later reading. If a topic interests them and they don't have time to read it right away, they'll post a ping to their own screen name. Usually a ping is directed to another Freeper in the To: line when posting a comment.
Bump or bookmark are also commonly used terms when referencing a thread.
It serves as a bookmark of sorts. When you click on 'my comments' the ping will take you back to the thread you were interested in earlier.
I hope my explanation wasn't too vague. Please feel free to ask more questions. I'd be happy to answer them. :)
I just used the reply line in posting comments to ping you, and myself, to this thread.
This is a very useful thread you might enjoy.
http://www.freerepublic.com/focus/f-news/1689962/posts
Happy Freeping! :)