One of the funniest idioms in German is, "Ich verstehe nur Bahnhof" (Literally translates to: I only understand train station) meaning 'I'm not quite following or don't understand'.
My guess is it's a reference to those speaker systems announcing things with a bit of static. Even if you understand German you might find the perfect time to use this saying after hearing some people with a strong Bavarian dialect.
There are more polite ways to tell someone what they are saying isn't so clear, such as a simple 'Wie bitte?', but using idioms is always a fun way of feeling like a local. Sometimes a little gruffness is crucial to survival here.