Look at the bottom of your stats screen, it tells you what each thing means. The blue triangle means that traffic is declining for that item. No triangle means that it is staying constant. A red triangle means that traffic is increasing. These are indicators not much more. Although interesting, I find that these change so often that they are not exceptionally important. They just give you a sense of trend.
Please see FAQ's http://hubpages.com/faq/#triangles