Maltodextrin is a natural polysaccharide, or complex carbohydrate, derived from starch and used as a food additive or thickening agent. It is easily digested and can be either moderately sweet or without much discernable flavour. It is widely used as a sweetening ingredient in many processed foods, sweets and soft drinks, and as an energy supplement for athletes and body-builders.
Which starch is used depends on its origin; in the US it is usually corn, while in Europe it is more likely to be wheat, leading to possible concerns for those suffering from gluten-related conditions, but maltodextrin is highly unlikely to contain enough gluten to have any effect.
Maltodextrin is much favoured by athletes, because it can provide a burst of energy very quickly during exercise and doesn't slow the body down. It has a high glycemic index which can help provide a constant level of energy release and a fast absorption rate.
Another kind of 'resistant' maltodextrin turns regular maltodextrin into a starch that resists digestion, and so passes directly through the upper intestinal tract. It is then fermented by bacteria in the lower intestine, producing energy and some fatty acids which help cells remain healthy in the colon wall.