What's the difference between filling and frosting? Does filling mean filling inside layers of a cake and frosting is covering the outside of the cake?
Pretty much. A "filling" is any substance inside of a pastry or pastry-like food item (a filling inside a sandwich, a filling inside a cinnamon roll, a filling inside a pie, a filling inside a calzone). So when you're talking about cake, anything between layers is a filling: more frosting, a layer of jam, maybe some ganache.
Frosting is a specific type of thing that's used to cover the outside of a cake. There are many kinds (contrast buttercream to whipped), but is generally more substantial than an icing, which also can be used to cover the outside of a cake. In general, things on the top of a cake are called toppings, in the same way that things on the top of a pizza are called toppings, but that usually doesn't count the frosting or icing or glaze.
You can therefore have a filling made of frosting, but you cannot have a frosting made of filling.