What does dog-to-dog socialization mean?

​What does dog-to-dog socialization mean?
This picture right here is a group of dogs showing excellent socialization skills. Note, none of them are interacting with each other, they are interacting with their owners. Proper dog socialization means that your pup is able to think and work despite being in close proximity to other dogs.
The approach of "socializing" meaning letting dogs play with each other makes for dogs with lower impulse control and a propensity for reactivity. Let me explain. A dog that is taught from square one that other dogs are SUPER EXCITING is going to get SUPER EXCITED every time he sees another dog. This excitement can include barking, howling, jumping, and lunging in desperation to see the SUPER EXCITING THING. The ears shut off and you've just lost control of your dog. There goes their recall, there goes their leave-it command, there goes everything you thought your dog was good at when at home. As they grow older this excitement can slip into the realm anxiety which in turn breeds reactivity. A dog who always feels TOO MUCH come across as too much, other dogs may not appreciate that which in turn could cause retaliation, and then teach your dog that social interactions are bad. But he is still too excited and now unsure, which often manifests as fear reactive behaviors.
On the flip side, the approach I like to teach dogs and owners is that your number one approach to any dog-to-dog reaction should be a calm and relaxed mindset for both you and your dog. Dogs should learn to look to you for guidance, direction, and permission in social interactions with other dogs. They need to learn how to exist near other dogs long before they need to know about playing with them. In classes we don't encourage dog-to-dog interactions because we want to be the lighthouse of calm in a sea of canine chaos. Once they have learned the art of being calm and listening to you in intense environments, only then can we open the door to fun activities. We can introduce them to dogs that we know, dogs we have met before (maybe in obedience class) so they feel confident and you can feel confident you aren't running into a potentially reactive dog. Let them play but ask them to check in (and reward them for that) so that they remember you are there and they remember that you are their number one. A calm well balanced dog handles life a lot easier and is prone to fewer issues.