A low-cut filter is an electronic filter that allows only the frequencies higher than a given point to pass, eliminating all frequencies below this point. In actual use, it has an effect similar to that of using a shockmount, in that it eliminates low-end rumble and similar noise. Low-cut can be used when recording acoustic guitar and, sometimes, vocals to alleviate low frequency noise that can seep into your recording.
Low-cut switch engaged