The nine stripes are represent the letters of the word "freedom" (Greek: Ελευθερία). White for Freedom and Blue for the Greek Blue Sky. The cross on the left is the sympol for the Eastern Orthodox Christianity.
WHAT IT MEANS TO BE GREEK TO BE GREEK - A Fantastic Video (Needs some editing to show it to kids. but there's a lot of really awesome stuff to think about what it means to come from a culture that old)