What’s this icon for?
 

Its a "Slow Down" warning. Either you are driving too fast for the road speed and conditions, or a reduced speed limit is up ahead, or a curve in the road is going to put you in a ditch. In my experience its usually the last two scenarios when that happens for me.

Ahh, thanks! No wonder the way I drive, I get this way too often :)

Actually, I dont think I've seen it when I'm speeding on a road where the limit isn't about to change (maybe if going way too fast). Its mostly the last two situations mentioned

It's a "predictive efficiency assist", as BigKutta mentioned it gives you a warning about some sort of "situation" ahead that requires you to slow down - or as Mr.T would say: take your damn foot of the accelerator fool !!! The "idea" is that when you drive predictably, you drive more efficiently - yeah right ...........

I think its telling you to coast to save gas. I vaguely recall some language like that in the manual, it is in the manual though. I'm too lazy to look it up (in my car or online)

Every time I've gotten it has been when I'm going down a hill with my foot completely off of the accelerator, so the icon always counter-intuitive. But yeah, it's just telling you that you should slow down because of something coming up on the road - a sharp turn, for instance. If you have Adaptive Cruise enabled, it will actually slow the car down when it thinks it needs to, but if you don't have it on, it just shows the icon without taking any action.

Simply put ... get your foot off the gas! ;)