"In ancient books of Asshai it is written that there will come a day after a long summer when the stars bleed and the cold breath of darkness falls heavy on the world. In this dread hour a warrior shall draw from the fire a burning sword. And that sword shall be Lightbringer, the Red Sword of Heroes, and he who clasps it shall be Azor Ahai come again, and the darkness shall flee before him."
The bleeding star(s) probably refer to the red comet, and the cold breath of darkness most likely to the Others and the approaching winter. The prophecy speaks of the return of Azor Ahai, to once again drive back the Others.