Two short lines hold one surprising picture. Looking down on a rainy street, or perhaps just across a crowd, the speaker sees umbrellas and thinks of a city skyline. The rounded tops drift along at roughly the same height, like domes on rooftops, except these buildings are walking.
The title does most of the work. Without "Skyline," we would simply see umbrellas; with it, the scene turns into a small, shifting city made of strangers. The word "moving" matters too, since skylines normally stand still. Here the whole horizon flows and rearranges itself as people pass.
The speaker doesn't state a feeling, but the tone seems quietly delighted. There is a gentle, observant mood, the pleasure of noticing something ordinary and seeing it fresh. The poem suggests that a wet, grey moment can hold a bit of beauty, if you look at it from the right angle. It also hints at how a crowd of separate people can form one connected shape.