Aggression is "intent to do harm." Most of the time in dogs, barking, growling, and even biting are ritualized ways to get another person or dog to go away. This video gives you a starting point on what to do if your dog is showing signs of aggression or 'reactivity.'
Note: this could be a life-and-death situation, depending on the severity. As always, please consider hiring a dog trainer near you if your dog or anyone else is in danger.