What is a reactive dog?

"A reactive dog will rush toward something or someone that he is uncertain about, barking, lunging, growling, and making a big display. People sometimes perceive reactive behavior as aggression, but a reactive dog is not rushing in to do damage; he is attempting to assess the threat level of a given situation. ... Reactive dogs are anxious, and their response is intense because they are freaking out." Leslie McDevitt, Control Unleashed