Multiband compression splits the signal into 2–5 frequency bands, each with its own compressor settings. Use cases: taming a boomy bass without affecting the midrange, controlling harsh cymbals without dulling the snare, de-essing (compressing only the sibilance range around 5–10 kHz), and mastering — where subtle multiband compression can tighten a mix without audible pumping.
The risk: poorly set crossovers between bands create phase issues and an unnatural sound. Multiband compression is powerful but easy to overdo. Most mixing tasks are better served by broadband compression + EQ. Multiband is a specialized tool for specific problems.