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.

FODMAP stands for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. FODMAPs generate gas as side effect of being fermented in the gut. Most people just pass this gas, but for some people, usually people with irritable bowel syndrome (IBS), it can be very uncomfortable and amplify their other IBS problems.

People who are suffering from pain and bloating with no obvious cause may be advised to go on a low-FODMAP diet for a few weeks to see if their symptoms go away.

We just had a story about de-farting beans on the front page. The FODMAPs are (among other things) the bean farts.