Low Pass
This filter will allow only frequencies below a certain frequency to pass, thereby removing higher frequency audio. This filter is useful for removing high-frequency noise due to artifacts from the digitization process. You can also use if you know your source material doesn't have any audio content above a certain frequency, and you want to remove the noise above that frequency. You can also use this filter to create certain effects. One effect is to make the audio sound like it is being played far away and muffled by walls and other things between you and the sound source.
The frequency control determines the cut-off frequency, or the point at which the curve hits the -6 dB level, and the "Steepness" control determines how many samples the filter uses as input, indirectly affecting how sharp the drop-off is at the cutoff frequency. You can use the graph in this dialog to see how the filter affects different frequencies.