A lot of fearful, timid, anxious, reactive, and aggressive dogs are very easily spooked by little things in their environment.
When they are out and about, they would focus intensely on everything around them, well, everything except what they should really focus on, which is their human owners. That is why we see so many dogs on the street barking, lunging, growling, and just going crazy at the end of the leash when their owners are trying their best to keep things under control to no avail.
It is as if the dog is looking at everything on the street with a magnifying glass and turning every little thing into a very big deal. The dog would tunnel vision on things like noises from children behind the fence, a bus, a bicycle, a bird, a mother pushing a stroller, a garbage can, a postman...etc and react to them from afar in an aggressive manner.
Interestingly, if you watch these dogs who seem to have such keen observation to everything around them, they hardly pay much attention to their own humans.
This is actually the exact opposite of what we want.
What we want is a dog who is indifferent and impartial to all the little things in their surrounding but very in tune with their owners.
We want environmental blindness.
What we want is to teach the dog to focus on what is important (the owner) and not make a big deal and waste his time on things in the surrounding which are actually irrelevant.
We want to teach the dog how to prioritize properly so he will always look to his human for direction, wait on his human for permission, and lean on his human for protection.
The dog should look at his human owner with a magnifying glass and always pay attention to what his human wants while turning a blind eye to all the unimportant details in the background.
This is the essence of environmental blindness.
When you are very relevant to your dog, when you are in a position for your dog to follow willingly, when you are whom your dog cares about the most, you will become a big deal to your dog and everything else will no longer matter that much by comparison. That is when your dog can be blind to the environment and stop reacting to all the silly little things.
Hope this makes sense.