The German word Abmeldung means "Deregistration".
When becoming a resident of Munich, or indeed anywhere in Germany, you must register your address with the authorities within 7 days of arrival.
When leaving you have to deregister again.
In Munich the registering and de-registering is taken care of at the Kreisverwaltungsreferat, or KVR for short, at Poccistraße.
For more details on this see: Residence registration