It means the sun is shining too brightly. Park the car in the shade for a while, and the warning light should go away.
Ha ha ha, just kidding, welcome to UnitedBimmer
It's the brake lining wear indicator, telling you that you need new brake pads. You'll need new brake wear sensors too, since the light came on. But they don't cost much. Pick them up from the dealer, or buy them online, and have your mechanic slap them in