"Faint" is really two different adjectives. As Karl showed, the more common version of "faint" essentially means "barely perceptible". A second, mostly unrelated meaning of the adjective "faint" comes from the verb "to faint": 昏厥, lose consciousness. If someone feels faint, they feel like they are in danger of fainting. Fainting was much more common in the past, or at least it was portrayed as being more common in literature. One reason for this was the higher rate of respiratory diseases like tuberculosis, and the fact that upper-class women wore corsets that prevented them from being able to breathe easily. China had footbinding, and the West had corsets. If a woman wearing a corset exercised, or if she experienced a strong emotion, she wouldn't be able to breathe naturally to compensate, and might faint.
Now, we literally say "I feel faint" if, for instance, we are running a marathon, have a salt imbalance, and almost pass out. We will also occasionally use it when we experience a strong emotion that we don't know how to deal with. Emotions like that don't actually make many modern people lose consciousness, but they might have done that to a tuberculosis patient or a woman wearing a corset in the past, and most people in the west will have read old stories where people pass out because of strong emotions. Thus, they occasionally use "I feel faint" as an expression, or when writing their own stories, even though it is not entirely accurate anymore.