Maltodextrins are a group of mostly unfermentable carbohydrates produced by the partial hydrolysis of starch or glycogen. Maltodextrins typically impart little or no flavor upon the finished beer, but are important because they can be a valuable method for adding gravity and perceived body and mouthfeel to a beer.
This can be extremely helpful when you are brewing a heavy adjunct beer, such as a gluten free ale, that might have a thin or diminished body. Maltodextrin is often made from corn, and a typical composition will be .5% dextrose, 2.5% maltose, 3.5% maltotriose, 93.5% higher saccharides. You will want to consult your vendor for actual numbers.