Some people may have a hole in their throat just below the Adam's apple. This is their airway i.e. they breathe through this hole. The stoma is done surgically because of cancer or because of physical injury to the area. We never perform this on the field, it is much too dangerous, despite what we might see on television.
If, as a first aider, you ever need to perform rescue breathing, it is in this hole that you would need to blow in to. It may sound odd, but it does work. And there is one major advantage over breathing in to someone's mouth; the air can't get in to the person's stomach because this opening only goes to the lungs. And if they do vomit there will be no interference with their airway.
The stoma may be held open by a plastic ring. Do not remove this ring. Some people with a stoma may keep it covered with a light scarf, so you need to check for this.