The Wikipedia page for it is pretty good. Basically, there are a number of short-chain carbohydrates that tend to pass through the small intestine (where nutrients are digested and absorbed into the bloodstream) and reach the large intestine (where water is removed). Bacteria in the large intestine eat these nutrients (fermentation). In some people, this causes intestinal distress. (Bloating, gas, discomfort, watery stool, etc.) It's not clear why this only affects some people.

You hear a lot about it because a large subset of people have discovered that a low-FODMAP diet relieves their torment of intestinal distress.