A filter which attenuates frequencies above its cutoff frequency (which is the point where the signal is 3 dB down). The slope of the filter is the rate of attenuation, i.e. 6, 12 or 24 dB per octave.

The cap in question is part of the guitar’s tone circuit, which along with a potentiometer forms a variable low-pass filter. A rich folklore has developed over the years around these circuits ...