Lowpass filters block high pitched frequencies (treble), but allow low pitched frequencies (bass) to pass. They can be used to reduce high end hiss noise or remove unwanted sounds above the given cutoff frequency. If you were to apply a lowpass filter with a cutoff frequency of 1000Hz on speech, it would make it sound mumbled and deep. Lowpass filters can also be used to eliminate aliasing when used before downsampling. This is an example of how to use a low pass filter to remove a high pitch noise from a sound clip.
This is the original clip with the high pitch noise in the background:
Looking at the clip as a graphical representation you can see the high pitch noise.
The straight line just above the middle is the high pitch noise. It's above the rest of the sound, so it can easily be removed by cutting off all the frequencies (sound) with the noise. Lowpass filters basically trim out what you don't want above a certain frequency.
After doing a 15000hz lowpass filter, the sound looks like this:
And this is what it sounds like: LowPass_filter_1_15000hz.wav (420KB)
All the sound above 15000hz has been removed. The high pitch sound is still there, but it's reduced. This means the high pitch noise was right around the frequency 15000hz. To completely get rid of it, I lowered the cut-off to 14000hz:
And this is what it sounds like: LowPass_filter_2_14000hz.wav (425KB)
Now the high pitch noise is pretty much gone.