In the UK, blue plaques have been placed on houses the signify someone famous has lived there. These could be artists, scientists, politicians or people from any group who have contributed meaningfully to society in some way. Chris is using $500 to parody these plaques with the message "An anonymous human once did a small, almost imperceptible, kindness to another anonymous human here." A tongue in cheek subversion of something known, to spark people's imagination about kindness (and what might have happened there).