The mouse over says that the ones attached to subject lines will take you to the first new post in that thread as a ways of reducing your searching time. The ones on the far right take you automatically to the last post in that thread regardless of whether you have read any of the thread at all.
I'm not convinced that the downward arrows mean new stuff. I think the side pictures, the dark, closed envelopes verses the brighter, open envelopes, are what signifies new messages.