This is the 635th most frequent German word.
müller
"Müller" means "miller" (a person who works in or owns a mill).
Here, 'Müller' is used as a surname, a common use in German-speaking countries.
Herr Müller arbeitet in der Stadt.
Mr. Müller works in the city.
In this context, 'Müller' refers to the profession of a miller, who operates a mill.
Der Müller mahlte das Korn zu Mehl.
The miller ground the grain into flour.
Here, 'Müller' appears as part of the name of a museum, possibly named after a person or family with the surname 'Müller'.