In a parametric EQ, Q controls the bandwidth (width) of the boost or cut. A Q of 0.7 (about 2 octaves wide) is broad and musical — good for general tone shaping. A Q of 5–10 (fraction of an octave) is surgical — for removing specific resonances. The relationship: higher Q = narrower bandwidth.
Q is also used to describe filter resonance in synthesis — a high-Q filter rings at the cutoff frequency, creating a resonant peak used for wah-wah and synth sweep effects. In room acoustics, room modes have a Q factor describing how sharply they resonate — high-Q modes are narrow but intense; low-Q modes are broader but less severe.