The Horn of Command allows you to control your hired units in various ways. It has infinite durability. When you first craft the horn, it will simply be named the Horn of Command, but when you right-click with it for the first time, you will be offered the choice of turning it into either the regular Horn of Command (Halt/Ready Units) or the Horn of Summoning (Summon Units). Because of this, it is best to have both types of horns in your hotbar when dealing with hired armies, unless you only have one or two units (that you could command in their individual command GUIs).