Its most common usage over here was, originally, to describe HM Queen Liz II when she condescended to mingle with the crowds of the great unwashed who flock to gawk at her when she does anything. I have a vague memory that the first time she ever did this was on a tour of Oz, back in the 70s, or thereabouts.
The phrase is now used for any "personality" appearing in public, who deviates from the route prescribed by the organisers of the event, and stops randomly to talk to members of the public.