a capacitor is actually a high-pass filter, which doesn't let lows through. when used as a buffer to ground, it only lets high frequencies pass to ground, so those frequencies get a massive drop in volume. a tone control is actually a volume control for high-end frequencies. to cut bass, the wire a pot so that the cap is in series with the signal (ie, the signal passes through the cap to the amp), and the pot varies resistance between the actual signal and the output, creating a volume control for lower frequencies, ie bass control.
