Slip leads for dogs, also known as slip leashes, as the name implies, are simply leashes that are meant to be quickly slipped on dogs. More specifically, they are composed of a loop handle and another loop at the other end meant to go around the dog's neck.
The loop meant for the dog's neck can be opened wide before slipping it around the neck. Once slipped over, pulling the leash will tighten the loop so that it fits well without sliding off.
Having a slip lead with a stopper is very important to prevent the loop from opening too wide and risking it slipping off the dog's head. Therefore, once the loop is placed over the dog's head, sliding the stopper helps prevent the loop from a widening or your dog from backing out from the leash. Ideally, you should be able to slide 2-3 fingers between the loop and your dog's neck.