Leave means "don't interact with that".
"That" may mean a person, dog, rabbit, horse or cat.
"That" could also be a dropped food wrapper or nice steaming pile of fresh horse poop.
"That" is simply the thing you would like your dog to ignore, come away from, drop and leave alone.
"interact" means look at, touch, smell, move towards, think about.
So an example may be to ask your dog to "leave" the child with an ice-cream. As we do not wish to pair a negative with the child we will use a neutral tone of voice when training this command. We will not use harsh lead or collar corrections either. "leave" will start to mean to your dog "better stuff with owner" meaning your dog decides not to mug the child and their ice-cream but to return to you in expectation of something better.
This exercise will also need to be repeated many times in order to create an automatic response in your dog, your dog "leaves" the thing you want it to ignore because they have repeated the exercise "leave" so many times.
This video is from Emily Larlham, it is one of the best methods for teaching "leave":