Cornstarch gives silkiness and body to a thin gravy or sauce.
A tablespoon of cornstarch generally thickens one cup of liquid. Too much cornstarch will make your sauce gelatinous; you're better off adding too little than adding too much. Avoid freezing sauces that you've made with a cornstarch slurry as they turn gritty when reheated. Add the slurry only in the last minute of cooking. The sauce will turn gluey if you cook it too long.