King Stannis’s new banner represents the fiery heart of the Lord of Light, R’hllor, whose religion Stannis is supporting. While Stannis is not particularly religious, the red priestess Melisandre of Asshai has promised him her support and has converted many residents of Dragonstone—including Queen Selyse—to the worship of R’hllor.
Information about King Stannis Baratheon that reveals spoilers from the books.