This is where language gets literal for once! "Prendre" means "to take", so the sentence, while not in standard English, does make sense: "What do you take for breakfast?" "Au" does replace "à + le", meaning the complete sentence would read more like "What do you take at the little lunch (breakfast)?" which in France is a completely correct sentence. They're just little colloquialisms they use that don't look completely right to our English-accustomed eyes!