Comb filtering occurs when a sound reaches your ears directly AND via a reflection (off a desk, wall, or ceiling) a few milliseconds later. At frequencies where the two signals are in phase, they add (+6 dB). Where they are out of phase, they partially cancel (−20+ dB). The frequency response graph looks like the teeth of a comb — hence the name.
The desk reflection is the most common comb filtering problem in home studios: sound bounces off the desk surface and reaches your ears 2–5ms after the direct monitor sound, creating a deep notch around 1–2 kHz — right where vocal presence lives. Fix: angle monitors upward, use absorptive desk pads, or position monitors on stands behind the desk.