to compare how a person rates themselves with how good they actually are.
I always remember an incident at University. I was at a pub, and I overheard a conversation where some guys who were doing the same Engineering course as me were talking to some local girls.
One of the girls was trying to show how smart she was compared to the University Student. "You think you know it all", she told the Student. "I work at Dixons (an electrical retailer), and I had to learn how to use a Video Recorder in a morning"
Charles Darwin says it well "ignorance more frequently begets confidence than does knowledge"