Top Butt is in reference to the upper portion of a woman's ass. Most womens' upper asses are slightly to largely flat, which contributes to the appearance of a sub-par ass. Top Butt signifies that there is ample meat in the upper regions of a woman's booty, resulting in a fully hemispherical and beautiful ass.
Example 1: My God, her ass is heavenly. Aesthetically, this is due largely in part to the existence of top butt.
Example 2: Her butt is juicier than a t-bone steak, thanks to that top butt.