FODMAP is an acronym for Fermentable Oligo-, Di-, and Mono-saccharides And Polyols. (You can see why they went for the short version!).
The FODMAP diet is more properly called the Low FODMAPs diet, because the idea is to limit your consumption of these nutrients.
All of these nutrients are in the carbohydrate family; some are sugars (such as lactose and fructose), others are sugar alcohols (such as sorbitol and mannitol), and some are non-digestible fibers (such as fructans and galactans). All occur naturally in whole foods such as fruit, dairy, beans, and grains. Sugar alcohols are also used in more concentrated amounts in food processing to produce sugar-free and diabetic foods.
Below are articles about FODMAP for further reading.