What are FODMAP?

Put simply, FODMAPs are short-chain carbohydrates (sugars) that are not properly absorbed in the gut. When FODMAPs reach the small intestine, they move slowly and attract water. When they enter the the large intestine, FODMAPs are fermented by gut bacteria, causing gas to form. The extra gas and water cause the intestinal wall to stretch and expand. Because people with irritable bowel syndrome have highly sensitive bowels, the stretching of the bowel wall causes excessive pain and discomfort. FODMAPs occur naturally in many foods and food additives.
What happens when you consume FODMAPs?
FODMAPs are found in a wide variety of foods, and most people eat foods high in FODMAP on a daily basis without experiencing any problems. When FODMAPs are eaten, the gut bacteria use the FODMAPs as a source of fuel in order to survive. The bacteria rapidly ferment the FODMAPs, producing gas in the process. 

These processes occur in all people (with and without irritable stomach / irritable bowel). The difference is that people with irritable bowel syndrome may have problems with motility (the speed at which the contents move through the bowel) and / or a highly sensitive bowel wall. The extra water and gas in the intestines causes the intestinal wall to expand, causing the usual symptoms such as pain, bloated stomach, bloating, nausea, gas, and digestive problems (diarrhea, constipation, or both).

What are FODMAPs in? 
As mentioned earlier, FODMAPs are found in a wide variety of foods, such as garlic, onions, fruits, vegetables, bread, cereals, nuts, legumes, and confectionery. It is impossible to guess the FODMAP content of a product. Instead, a careful laboratory analysis is required to understand the FODMAP content of foods. The Monash University team is an expert in such analysis. The information about the FODMAP content of foods is published through their app, the Monash University FODMAP Diet App. The app uses a simple traffic light rating system to indicate whether a food is low, medium or high FODMAP and in what amount it is safe to eat.
Content was taken from Monash University (https://www.monashfodmap.com/about-fodmap-and-ibs/)