Brake chatter is a rapid, stuttering vibration or grinding-flutter you feel and hear under braking - distinct from a smooth squeal or a slow pulsation. It usually traces to the rotor surface or loose hardware. Here is what causes brake chatter.
Rotor surface issues
The most common cause is an uneven or improperly-finished rotor surface - thickness variation, or a non-directional finish that was not applied, causing the pad to grab and release rapidly; see brake judder. A rotor with hard spots from overheating - see blue hot spots - chatters as the pad crosses the harder areas.
Loose pads or hardware
Pads that can move in the bracket because of worn or missing anti-rattle hardware can chatter and clatter under braking; see anti-rattle spring and brake pad gap noise. Fresh clips and shims hold the pad snug and stop the chatter.
Bad bedding or cheap pads
Improperly bedded pads leave uneven deposits that cause a stuttering grab, and cheap pads with inconsistent material can chatter from the start; see how to bed in pads. A glazed pad can also chatter - see glazed brake pads.
The fix
Address the rotor (resurface or replace as a pair, clean the hub, torque lugs), fit fresh anti-rattle hardware, use quality pads, and bed them in properly. That combination cures nearly all chatter. Fit quality pads and a full hardware kit.