They are an indication of intestinal worms and the general health of the large intestine/colon. This comes from Chinese medicine - muck like how we can look at the tongue to determine the health of the liver and digestive system or how in reflexology, points on the hands/feet correlate to different body parts - the face does as well. Another example is horizontal lines across the bridge of the nose, when you don't scrunch your nose as a regular practice, can indicate pancreatic insufficiency.