Caramel Sauce for dipping apples will thicken as it cools.  In order to get it back to it’s silky smooth self, you can either microwave it for about 20-30 seconds at a time, stirring in between, or reheat gently in a saucepot with a couple tablespoons of heavy cream.  The goal isn’t to get the dip hot but rather to loosen it up so that it’s a good consistency for dipping.