Mahnbescheid (court payment order)
A Mahnbescheid is a letter from a court. Someone uses it to demand money from you.
A later Widerspruch still counts as long as the court has not issued a Vollstreckungsbescheid (§ 694 Abs. 1 ZPO). But from this day on, the creditor can apply for one. So don't wait.
What you can do
If you do not owe the money, or not all of it, file a Widerspruch. You have two weeks from the Zustellung (formal delivery).
A form for the Widerspruch comes with the Mahnbescheid. Send it to the court, not to the Gläubiger.
If you do nothing, the Gläubiger can apply for a Vollstreckungsbescheid. With it, they can for example have your bank account seized.