My German is not good enough to explain this and English is my first language, but this is my understanding:
Tu manges du pain = Du isst Brot
Tu manges le pain = Du isst das Brot
Tu manges un pain = Du isst ein Brot
N.B. "du" is short for "de le", so you will also see "de la" and "de l' " in this situation for other words. For example, "Tu manges de la nourriture".
In French you can't put a noun with no article, so where German and English would say "Du isst Brot" or "You eat bread", in French you have to add the "de" plus the definite article. In this case it's "du" because "pain" is masculine.
Hope this helps.