"Reactivity" is a popular blanket term to describe dogs who are less than calm and collected when passing other dogs, strangers, cars, and so on. They may exhibit this behavior toward a particular trigger or toward several triggers. The behavior might include barking, lunging, growling, snapping, whining, and vocalizing. Why do they do this and what can you do about it?
This behavior is driven by emotion, not lack of manners, so all the manners training in the world is unlikely to be effective enough to change this behavior. You have to get to the root of the emotion to change the behavior. The emotion might be fear, and the dog is making a big display to drive the scary thing away. The emotion might be frustration - the dog might want to get to the thing, but the leash prevents them from moving the way they want to. The dog might be guarding you if they tend to guard resources. The dog might only act this way in the yard, and there could be a territorial element, or the fence could be adding frustration. If you use a shock barrier of some kind, that equipment tends to create this behavior by pairing scary warning tones or painful shocks with passersby.
What we do about it can include a few different approaches, but we have to manage the behavior and attempt to prevent the behavior while working to change the emotion.
NOTE: There are special classes for dogs that react to other dogs. A well-run class affords the option of seeing a wide variety of trained helper dogs in a controlled environment which are crucial elements of effective modification of this behavior to generalize the concept to all other dogs. This is not a group of dogs all in a room barking at each other by any means, it's carefully choreographed. These classes are typically called Reactive Rover or Kranky K9 or GRR. Classes are offered periodically through these companies and they use the least intrusive, minimally aversive techniques. That's important because adding aversive collars or punishment may temporarily suppress behavior but generally adds fuel to the fire in the long run. The problem with working privately is you forgo the controlled environment, and the trainer has to bring multiple helper dogs and also an assistant to handle the dog if the distance isn't fairly small since they can't see what's happening and coach well from a distance. You may end up on headsets and the trainer may miss fine details of your dog's body language at bigger distances. Adding an assistant makes the price astronomical for most people and makes scheduling tricky.
To prevent behavior, sometimes we have to take walks at quiet times of day or in quiet locations. Sometimes cemeteries are a great option. We can skip walks and stay in private yards, and if you don't have a yard, check out Sniffspot.
To manage behavior and distract the dog when we are out and about and a trigger pops up, we can use a few different techniques. One simple thing to do is a magnet hand - just hold a fistful of food in front of your dog's nose and start walking away from the trigger. The food must be high-value and smelly.
Here are some great distraction techniques, and you can find specific videos and articles here if you don't know how to play these games already.
Attention - get the dog to look at you
Find it - get the dog's head down
Pattern Games- get the dog to play a game like 1, 2, 3 walking or up, down
Treat Bomb - get the dog's head down
U-turn - go the other direction quickly
When we want to work to change behavior, we need to get specific.
What is the exact behavior you see?
What is the distance away from a trigger when it happens?
What the trigger is doing when it happens?
Where does it happen? Where doesn't it happen?
Is the dog on leash? Not on leash? With anyone or specific people or alone?
And so on!
Once you get specific, now you choose a location to practice where you think you can see triggers without eliciting a reaction. I like to go out to a busy park but way out in a field where I can see a trail from a distance. I know I am in the right place when my dog can notice without reacting, can eat without using extra pressure with his teeth, and can easily look away from the trigger. These are the techniques that begin to change emotions, and you can find specific videos and articles here if you don't know how to play these games already:
BAT / CAT - you need to have a good eye for body language to do these games
Counterconditioning & Desensitization - the foundation of most games, a good place to start
Engage /Disengage
Mark & Move
Pattern Games like 1, 2, 3 Voluntary, Superbowls (more for dogs that are fearful than for dogs that want to rush to the trigger) and Look at That or LATTE (all dogs)
Having help from a trainer who specializes in behavior will make these games go much more quickly, so reach out if you need assistance.