Pressing the Master button on the transport bar activates the master track, which is essentially, a programmable master TEMPO track, though it does do some other things. If you click it and it sends your song automatically to a tempo of 120bpm, then your MASTER track is set to that. Do a search in your manual for "master track" for more info.
Deceptively, it has NOTHING to do with mastering. There is no EQ, faders, FX, etc. to be found there.