In an army, an adjutant is an officer whose job is the office work of a unit: orders, lists, letters, who is where, what has to happen next. They are not usually the person leading soldiers in a fight. Their work is to keep the unit running on paper and by message, so the commander can think about the bigger picture. It is a real role in British and some other armies, and to people outside the army it sounds formal and a little old. You mostly meet it in military stories, news, or history, not in everyday talk. Though actually, English also uses the same word for a large stork — named that way because it walks on the ground in a stiff, soldier-like way. That bird sense is separate; the meaning here is the army officer.