Maltodextrin is a commercially produced substance, manufactured in a powder form, that is added to beer to increase the level of dextrins. They are non-fermentable, flavorless, colorless, and non-caloric, but contribute to the body of the beer, as well as its mouthfeel and head retention.
When too much sugar is added, the yeast isn't able to convert it all to alcohol. The beer will end up tasting sweet like a cider. You can try to fix the issue by adding more yeast to your homebrew. The extra yeast will help to finish fermenting that residual sugar.