I thought the captions made it clear, but then I reread them and it still doesn't help, lol.
Her ability was to see the possible outcomes and consequences of all of her actions, even ones as simple as flipping a light switch. Her mind was so preoccupied with multiple futures that she didn't pay attention to her body at all, leaving her in a state that appeared to be a vegetative coma and leaving others to have to feed her and clean up after her.
When she saw M-Day and her own depowering coming, she finally decided to act, writing down as much of her prophecies as possible, without stopping for a moment to eat, sleep, or anything, so she died.