The combo is available starting August 31, 2015, at participating locations, and includes half a normal order of a pepperoni Deep Deep Dish pizza (i.e. a Deep Dish Pizza, which comes out to 4 slices rather than 8) along with an order of Italian Cheese Bread (10 pieces) and a cup of Crazy Sauce (pizza sauce) for dipping.
The $9 comes out to about the same if you were to buy the items in piece meal (at least in my area) and is more of a choice that gives you more options in case you don't want to get a whole Deep Dish pizza (they don't usually sell half the pizza, except in a lunch combo along with a drink for $5 at participating locations) plus Italian Cheese Bread (their cheese bread is my favorite version out of the big chains; my review here).
Little Caesars $9 Box Set will be available for a limited time only.