It means that the narrow gap is just about the same size as the wavelength(please note that you spelled it wrong) of the wave. Whether it(the narrow gap) count's as narrow or not depends on the wave.

逼窄不窄跟逼没关，跟波有关。

山不在高 有仙则名
缝不在窄 卡波就行

衍射实验狭缝的要求

我真是渣渣
------一种窄间隙是一个差不多大小的波长的wave.so是否有差距是缩小或不取决于波。