The "Mastering" panel allows you to configure the global sound of your score/tab. By default the Mastering panel has two types of effects: a limiter/compressor and an equalizer.
The limiter/compressor lets you set the attack, gain and volume levels of the score/tab. The equalizer lets you play on all the frequencies of the score/tab to transform the sound as you like. Note that the "Mastering" panel also lets you add a reverb to all the tracks of your score/tab.