Reactivity ≠ aggression.
Reactivity is often lumped in with aggression and dogs can and regularly do express both, but just as commonly dogs are reactive without being aggressive.
For example, a dog who is pulling on the leash and lunging at people and/or dogs but he just wants to go say hi or play. That is still reactivity! Even dogs who are completely friendly can still be reactive; it's a very common thing for dogs to experience. Especially when a barrier like a leash or fence is in the picture but also just to noises or other random stimulus in the environment.
Reactivity doesn't make dogs bad (and neither does aggression, necessarily, but that's a topic for another day) but it can really put a lot of pressure on us humans in the situations in which our dogs are reacting. It can be embarrassing, frustrating, and downright discouraging! Having tried and true methods to work your dog through their reactive episodes and training to default to is important. Nothing feels worse than not knowing for sure if you're doing the right or wrong thing! So it helps immensely for both the dog and the handler when you have something certain to do when the going gets tough, because if we're stressed out, our dogs can perceive that and it can be a vicious cycle.