The Latin motto beneath the shield–“Miserando alque eligendo”–means “Having mercy, he called him.” It is a reference to the account of Jesus calling the tax collector Matthew. The motto refers to the call the pope felt to enter the Jesuits, which occurred on the feast of St. Matthew when Jorge Bergoglio was 17 years old.
UPDATE: For more on this, see “Is the Pope Still a Jesuit?” from America magazine.