What does this traffic sign indicate?

You answered correctly. This yellow rectangular sign with a black border and number indicates a federal road (Bundesstraße) in Germany, specifically road number 35. It does not represent a speed limit or a minimum speed. Federal roads are important national routes, and their signage always uses this yellow design with a black number. Recognizing these signs helps you understand which type of road you are on, which can affect other traffic rules like right of way and permitted vehicles.