A sick note (Krankschreibung, Arbeitsunfähigkeitsbescheinigung, AU-Bescheinigung or gelber Schein) is a document you get from a doctor. It confirms that you are sick, and that you should get sick leave.
Most German employers ask for a sick note if you are sick for more than 3 days.1
Since 2023, sick notes are digital. You get a paper copy. Your health insurer gets a digital copy from your doctor. Your employer gets a digital copy from your health insurer. Only you get a paper copy.2