Multiband compression divides the frequency spectrum into different sections, or bands, so that each has its own unique compression settings. This allows a longer attack time for the low band of that bass drum to punch through, while keeping a shorter attack time in a higher band to keep the guitar in check.

A multiband compressor allows you to split a track into different frequency ranges (called “bands”) and compress them independently. You can choose to compress only a certain part of a track's frequency spectrum, or apply different flavors of compression to several areas of the spectrum.

Why do we want to use a multiband compressor in audio mastering? Because it'll give you more control over the dynamics of the audio signal. With this you can process the low end of the mix without even touching the cymbals. This is why it is a very useful tool.