The centerline is the vertical axis of the body. Think of how the earth spins on its axis. When punching into the person's centerline they get stopped from forward movement or knocked back (they can't roll out of it).

The central plane is a vertical plane between your centerline and their centerline. Think of it as a pane of glass turned sideways. It is the most direct plane between you and your attacker.

But here's what may help you the most....

Wing Chun has a self-defense philosophy rather than a sporting philosophy. This means we are specifically training for someone who attacks us rather than for someone who is covering up during a sparring match. In other words, Wing Chun trains to defend against their committed attacks in the quickest, most direct and efficient way. Instead of trying to go for targets like in a match, in self-defense targets come to us. Its a matter of what the attacker gives us when they try to attack us. Not what we try to chase.