In audio this term is used to refer to frequencies that are on the edge of, or just beyond, a selection. In frequency response curves, for example, the skirts are the range of frequencies just beyond the rated response. In analog systems these frequencies are still reproduced, but are at a low enough level that they aren’t considered part of the official frequency response. Another example would be an individual EQ filter. The Q will determine how wide a range of frequencies are effected on either side of a selected cut or boost frequency. The bandwidth is the range of frequencies that are significantly effected (“Significantly” may mean different things depending upon the specific context, though the point where the response or signal is -3 dB from the rest of the response is often considered the cutoff frequency), and the skirt is the range of frequencies beyond the bandwidth or cutoff frequency that are still effected to some degree, but not enough to be considered part of the bandwidth.