Lactose intolerance is what happens when you lack the capacity to break down lactose (a type of sugar) in your gut (intestine).
Sugars can consist of many subunits, but the small intestine only absorbs single subunits. In order to absorb many of the sugars, therefore, the intestine is lined with enzymes that break disaccharides (two sugar subunits) and polysaccharides (many subunits) into monosaccharides (one subunit). A person who is lactose intolerant lacks the enzyme (called lactase) to break down lactose (a disaccharide sugar) into its two monosaccharide components.
Thus, so far, we have a type of sugar (lactose) that cannot be absorbed by the bowel. The effects of this are twofold: firstly, the bacteria in the large bowel start gobbling up the lactose, and a biproduct of this is gas formation. Second, the lactose draws water into the intestine lumen ('inside') by osmosis, which can result in watery stools - diarrhoea. Both the gas and the extra water together can give you intestinal cramps too.
So, lactose intolerance isn't dangerous; it can just be uncomfortable. If you're coping with the symptoms, there's nothing else to worry about.