Cornstarch is a powder ground from the endosperm or white heart of maize (corn). It is used as an industrial anti-caking agent and as a thickener or binder.
As a thickener, cornstarch is actived by a) mixing with water and b) heating to 155°-165°. In my experience, cornstarch is not as robust in this role as wheat flour and if repeatedly heated and cooled, it will release and cease functioning as a thickener or binder.
To add to an already hot liquid, create a slurry of equal parts (by volume) cornstarch and cold water or milk. Then add it while stirring as vigorously as the dish will permit (without damaging vegetables or other structures in the hot dish).