mastering ,is in simple terms,taking all of yer different songs,..that peak in different places,...have different....frequencies,.....etc..levels,....and making thwem all sound.equal,.and uniform,..giving it a polished sound ,.a fattness,......without it,.....u may be turning up ,one song,.....after another,then turning,..it back down ,for another,.it may sound perfect on the recorder u recorded it on,.but.play it on another,.player,.etc,andu will see,how different,.in a nrgative way it would sound,.mastering,is a very important step,in getting your material,that in your face radio ready,sound,.peace