The multi-band compressor takes the signal and splits it up into as many as five separate bands of frequencies. Each band is then compressed separately. Sometimes the bands are compressed and then limited.
This maximizes the level separately in each band, so that when the signals are mixed back together again the result is a signal which is as high as it practically can be in every band of frequencies.
This does tend to change the balance of EQ of the signal, but it is a necessary price to be paid if ultimate loudness is desired.
It has to be said that there are now units on sale which claim to be 'mastering processors'. They even come with presets for quick results.
Mastering presets are to be avoided. Mastering is an extremely subjective procedure and very fine consideration must be given to every process involved. When done well, subjective loudness can be maximized with very little detriment to the overall sound quality.
When done via a preset, the result will be a signal that could have been louder, and could have sounded better.
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