Now, it means someone who's an example to the rest of us. It's not really used in normal converstation.
Originally, many years ago, charities like the March of Dimes would select a child for their advertising posters that were displayed during pledge drives. These "poster children" sometimes became mini-celebrities, recognized at charity events, etc. Eventually the term came to mean a typical example of something. It's usually used in a joking/sarcastic sense; if it seems like every time you go on holiday, everything goes wrong, you might tell your friends "I'm the Poster Child for Traveler's Aid."