The word Bad is German for bath.
All the ripples below her show she is bathing, in the Bath.
So in 1921 it was 2 Marks to go into the Baths there.
On a personal note, if I was around in 1921 I would have
paid her 2 Marks to watch. But that's just me!
Bath is a common feature of place names, indicating the presence of a bath, especially a spa.
As a protected predicate in Germany only towns with a state-approved spa may carry the prefix Bad.
Like the city of Bath in the UK got it's name from the Romans.
The picture of the bathing person pays tribute to that fact.