'Master' means the level of a master in that craft. Roughly speaking, someone who can perform that skill as their primary profession, is familiar with all aspects of that craft, and has nothing more they can learn from theoretical study. It is the skill level that if you were selling your skills in that craft on the open market, you would never need to exert yourself to find work. 'Grandmaster' would be someone who has reached a level of skill beyond that, and is reaching the edge of what is thought to be possible and starting to push that edge back.