Traditionally the multitrack recording is mixed-down to a stereo tape recorder. This stereo recording is then 'mastered', which usually means that EQ and (multi-band) compression is applied. When mastering for vinyl, EQ and compression had to be applied to keep the needle from jumping out of the groove.
Using MultitrackStudio, mastering after mixdown-to-stereo does not make sense, as all the settings involved in the mix are stored in the song file and can be recalled at any time. You can add effects to the mixer's Master section and use them for mastering purposes. Typically an EQ, a Compressor and a Master Limiter effect will be used. The Master Limiter should be the last effect. If multiband compression is required a Band Effect with compressors can be used.
When mastering a couple of songs that belong together (a CD for instance) it is very important that the songs sound 'the same'. They should be equally loud, have the same tonal balance etc. MultitrackStudio's songlist feature can be very convenient for this job. Using a songlist you can audition your CD before actually mixing the individual songs down to stereo.
The song can be mixed down using the Mix Down menu's "Mix down to audio file" option.