In general, mastering is the process of preparing a final mix for manufacturing. It is the last chance to add that final sheen to polish the sound. Typically, it is done on a stereo mixdown file and not a multi-track mix. At the mastering stage you add overall equalization, compression or other sonic enhancements to smooth out the sound. If the target media is CD, mastering ends with a final dither and resample to 16-bit/44.1kHz (CD sampling rate).
Many Sony users, including the authors, like to perform their final mastering in Sound Forge, but you can follow the steps in this chapter in ACID by rendering your final mix to a stereo file and then bringing it back into a new ACID project for mastering. Just add any of the effects ...