Officially, the name of the sign is “No overtaking of single-lane vehicles for multi-lane motor vehicles and motorcycles with sidecars”.
In concrete terms, this means that from traffic sign 277.1 all single-lane vehicles such as bicycles, mopeds or motorcycles cannot be overtaken by multi-lane vehicles – i.e. cars or trucks – and motorcycles with sidecars.
This overtaking ban is fundamentally based on Article 5(4) of the Road Traffic Act and is intended to protect single-track vehicles against the risk of local conditions when overtaking. This can be the case, for example, with bottlenecks, downhill and uphill sections or generally confusing traffic situations.
Failure to comply with the traffic sign can be punished with a fine of 70 euros and a point in Flensburg. You can read more about what the round, black and white traffic sign on the highway is here.