Ultima means "utimate", "last", or "final" or "end". The name has several different significances in the context of the novel.
Associating Ultima with the idea of being "ultimate" underscores the authority she carries both in knowledge and in the eyes of the community. She is an expert healer and her wisdom is not questioned by those who believe in her power and knowledge.
Associating Ultima with finality helps us to see that Ultima is the most important and the last teacher to guide Antonio to adulthood. She is his best and last real mentor, the ultimate authority for him on many things.
She is also part of a fading line of curanderas. Though the novel does not directly discuss the idea that these healers are on the wane, there is conversation about how people are becoming more modern in their views. Antonio's father, Gabriel, expresses this view in particular.
Finally, Ultima dies in the novel. The episodes of her life spent with Antonio and recounted in the novel are her "ultimate", final episodes. The is the end of her life.