Ass (pronounced with a short A, rather than the long A as in english) means Ace in German. (Before 1996 it was spelled As I believe)

Dusch Das is the brand name of the shower gel, Dusch means shower, Das is one of the three articles, so by turning the words around and into a rhyme, and says: "Dusch Das, the shower ace."

Perfect. Though, for a native English speaker, this commercial (and others of the same genre) left me with hard to resolve giggles. I still remember them fondly 20 years later. Also the German TV show Tutti Frutti (https://youtu.be/RZeQ-b1z_6c)