In the hair business, "damage" means that the cuticle is roughed up, cracked, chipped or even worn off. There's a long list of things that can damage hair: too much or too-vigorous washing, combing and brushing; too-hot dryers and styling implements; bleaching and dyeing; and perming and straightening are the most common. Even swimming in saltwater can do damage if you're not careful. But no matter what a product claims, damaged hair can't be permanently repaired. The only way to get rid of the damage is to wait for your hair to grow out and to develop healthier habits.
Meanwhile, there are ways to make your hair look less damaged. Avoid washing your hair every day (unless your scalp is oily); when you do wash it, use a conditioner that contains proteins and humectants. (If your scalp is oily, keep conditioners away from the roots.) When you dry your hair, gently squeeze out moisture with a towel -- never rub. A drop (just a little!) of silicone serum also can help smooth down the cuticle and add a bit of shine.