It shows that your sites traffic has decreased. For example if you see today's analytics then the percentage -44.80% is calculated from traffic comparision of current 30days (Sep. 3, 2010 to Oct. 3, 2010) vs. previous 30 days (Aug. 8, 2010 to Sep. 2, 2010). -44.80% shows that your last 30 days traffic has decreased by -44.80% compared to previous 30 days. Hope this example is not difficult to understand