Basically, you have an offensive target (i.e. whatever you're attacking) and a defensive target. You designate your defensive target before you actually begin to fight so that all heals and buffs automatically target that person without you having to pick them manually each time. Note that it doesn't actually heal them instantly, just targets them for you, so there's no issues if as a healer you have multiple people you're taking care of. It just defaults to whoever you set as the defensive target,