A skirt guard on a bicycle keeps your skirt, or coat, or child's feet from being flayed by the rear-wheel spokes.
My kids are big enough that I no longer ride them on the back of my bike, and I can't remember when I last wore a skirt and that's not just a memory problem, either. But I do commute on a bicycle, shine or rain. For rain I wear a rain parka. For heavy rain I wear a rain parka plus rain pants and overshoes.
The parka is long enough that it used to get caught in the rear spokes, which tended to mess it up.
Bicycle skirt guards are sort of under-represented on Instructables. 1lenore has a good one
using tulle, which seems to be a type of fabric. I wanted something a bit sturdier.
There is now a YouTube video that is sort of a survey of Japanese skirt guard designs.
Full disclosure: I did the video, and it's based on photos I took during my recent Japan trip, which came AFTER doing this instructable. If I redo my skirt guards, I think I may go smaller.