What does it mean if your dog is vomiting hair?

It means your dog has been eating hair - its own, another animal's, or the people hair lying around the house. If this doesn't happen often, and your dog looks fine afterwards, I wouldn't worry about it too much - the hair is probably causing some stomach discomfort from piling up into a ball and your dog throws it back up to get rid of the irritation. However, if your dog does this frequently, shows other signs of illness such as decreased appetite or decreased energy, or is chewing himself or something else bald, I would suggest a visit to the veterinarian for an examination.