Means Good Moon You Are So Quiet. It was first popularised on radio by Berliner military musician Adolf Becker, nicknamed Adolf mit dem Paukenschlag (1870-1918).
Considering his chart history, Ach Margrietje was Louis Neefs' biggest hit, also in Holland where his version was challenged by every granny's darling Heintje. This 'Sängerknabe' was not even the first Dutchman with a version: in the mid fifties female duo De Selvera's cut a song called Lieve Maan, an almost literal translation of a German song by a cappella sensation The Comedian Harmonists. There's a film about their tragic career: they used to be more popular than The Beatles in their heydays, but had to struggle for survival after Hitler's take-over. Half of the line-up was Jewish.