Frosting (it's called icing in the UK) is a food topping...usually goes on top of cakes. If you have ever had a cupcake or birthday cake they usually have creamy topping or a glaze over the cake and that is the frosting. Chocolate Frosting is chocolate flavoured of course.

This is a picture of a vanilla cake with chocolate frosting: http://www.glutenfreeheaven.com/~glutenfr/images/Vanilla%20Cake%20800.jpg