"Friend of the Court"?
 

I keep seeing the "Friend of the Court" title in stories, especially
involving Michigan. I'm not familiar with the concept. It seems that
people can pursue grievances for others, or give their opinion, even
though they have no personal interest in a case. Is that close? Is
this common to only certain states? I've never heard of it here in
California, for example. Can someone fill me in? Thanks.