A scoutmaster is a man who leads a group of Scouts — the adult in charge of the troop. He organises meetings and camps and looks after the young people in his care. The second half is master in the old sense “the person in charge,” not “someone who has mastered a skill,” and not 大師. Though actually, many groups now prefer scout leader, because master sounds male and a little old-fashioned; you will still meet scoutmaster in older books, in some countries, and in everyday talk. A woman doing the same job is not usually called a scoutmaster.