This is a summary, rather than a quote from the book.
Stannis says that Melisandre has shown him what will be happening in the flames. From a high hill in a forest he saw men in black behind a ring of torches, and shapes moving through the snow outside of it.
What Stannis has seen clearly appears to be the Night's Watch under threat from the Others at the Fist of the First Men, so clearly Melisandre can show the past or the present, or possibly the near future in the flames.