"Molding" is an old-fashioned training method where you physically maneuver the dog into a position in order to train it. The old "teach a dog to sit by pulling up on the collar and pushing down on the rump" is an example of molding.
Traditionally, dogs were taught "Down" (lie down) through molding. Techniques varied from "pinch the dog between the shoulder blades until he lies down" to "hold one paw and push the opposite shoulder until he lies down" to "pull straight down on the collar until he lies down."
It did work, but not very well. Molding turns the dog into a very passive learner, making the training process tedious and slow. For many dogs, it had the additional side effect of making training very unpleasant. The cumulative effect of being pulled, pushed, pinched, squashed, and knocked over in the name of training (especially on the "down" command) caused many dogs to show aggression, and these dogs were then promptly and unfairly labeled as "dominant."
To this very day, you will sometimes hear that "dominant dogs don't lie down on command" or "If you can't train your dog to lie down, it's because he's dominant." In most cases, this is a throwback to those old training methods and the aggression they sometimes produced.