A low-pass filter is an audio filter that cuts off frequencies that are higher than the desired cutoff frequency. This filter is popular in music production as it can be used to soften audio or remove undesired noise.
With FFmpeg the filter lowpass is used by setting the desired cutoff frequency in Hz. In this example, any frequency above 3200Hz is cut off:
$ ffmpeg -i input.mp3 -af "lowpass=f=3200" output.mp3
The low-pass filter is great for getting closer to the bass of the audio. For the opposite effect, a high-pass filter is used which can be found in question 36, “How to Add a High-Pass Filter to an Audio Track?”.