The course: how to make progressive house (ableton) does not say about the master channel. Why? Disappointed …
Mastering is pretty easy just adjust the volume levels of each of your tracks so each sit well in the master track. When i find that my tracks aren’t loud enough I slap a limiter on the master track. Adjust the ceiling and gain to taste.
There are much better ways to master a track especially in Ableton. Usually if you follow the tutorials well enough you won’t really have to mess with the master track too much.
Also sometimes its better to leave the master track untouched especially if your signed to a label. sometimes labels will send out your track to be mastered.