In the context of this quote, Celia has been separated from her lover, Gustavo. She is mentally and physically weary from depression. Neighbors had suggested remedies for Celia's depression. Others brought food to lift her spirits. Vilma Castillo tried to make a baked Alaska but set the kitchen on fire. The remedies ended up in flames, so to speak. Celia's aunt called a santera (a priestess of santeria) to examine Celia. The santera envisioned a "wet landscape" in Celia's palm, perhaps referring to Celia swimming in the sea at the end of the novel (and presumably the end of her life). Therefore, she would survive the actual flames (from the baked Alaska), the metaphorical flames (struggles with depression at being separated from Gustavo), and subsequent "hard flames" (hard times in her life).
The santera, in predicting that Celia will survive the "hard flames," shows foresight, literally and metaphorically. When Celia is in the asylum, she meets Felicia Gutierrez who killed her husband by burning him. Felicia dies; Celia believes the guards won't admit to giving Felicia a cigarette. The indication is that she killed herself. Nearly two years later, Celia names her new daughter Felicia, which Jorge thinks might be a curse.
Cursed or not, Felicia (Celia's daughter) does have a troubled life. She ends up burning her husband, Hugo (he survives), in response to his abuse of her. Felicia's second husband dies in a fire. So, there are quite a few "hard times" for Celia and her family (namely Felicia) which involve actual fire. So, the santera was referring to visions of "hard flames" and "hard times" that Celia would live through (survive). In fact, a few pages after this quote, Celia (upon discovering that she is pregnant), decides that if she has a girl, she would stay and teach her daughter to also survive the hard flames/times:
She would not abandon a daughter to this life, but train her to read the columns of blood and numbers in men's eyes, to understand the morphology of survival. Her daughter, too, would outlast the hard flames.