Lactose is a type of sugar and is made up of the two sugars galactose and glucose. It's found in milk from all mammals. Being lactose intolerant can be troublesome, although it is usually not directly dangerous.
In a crisis situation or when camping, of course, an upset stomach quickly becomes even more difficult. Both those who are lactose intolerant and the tent mate can then appreciate that a wide range of lactose-free freeze-dried food is available