Lactose intolerance, also called lactase deficiency, means you aren't able to fully digest the milk sugar (lactose) in dairy products. It's usually not dangerous, but symptoms of lactose intolerance can be uncomfortable.
The problem behind lactose intolerance is a deficiency of lactase — an enzyme produced by the lining of your small intestine. Many people have low levels of lactase, but most don't experience signs and symptoms. Only people with both low lactase levels who also have associated signs and symptoms have, by definition, lactose intolerance.
You can control symptoms of lactose intolerance by carefully choosing a diet that limits dairy products.