A high-pass filter is an audio filter that cuts off frequencies that are lower than the desired cutoff frequency. This is often used to cut out bass from an audio source while leaving the treble side of the audio.
The filter in FFmpeg is called highpass and is used by setting the desired cut off frequency in Hz. For example, here bass lower than 300Hz is remove:
$ ffmpeg -i input.mp3 -af "highpass=f=3200" output.mp3
The high-pass filter removes the lower frequencies but for the opposite effect, a low-pass filter is used. More information on the low-pass filter read question 35, “How to Add a Low-pass Filter to an Audio Track?”.