Magic Skirts happen when the local laws of physics which apply to a superheroine's skirt cause it to always cover up everything, regardless of what ridiculous things she may eb doing. Like (for example) falling feet first. Note that the skirt will always cover the girl's modesty from the "camera" (that is, the readers of the comic), and thus act as a form of censorship.
This is at least plausible if the skirts are quite close-fitting. It applies more to the pleated skirts which are in no way prevented from flapping around. Supergirl, we're looking at you.
Sometimes this is avoided by having the character wear something under the skirt, like bicycle shorts or a leotard.