This is a huge subject. Many trainers that have written books or have DVD's spend a lot of time going over this.

It basically means that the dog understands that the shock from the collar is not some random act of God. It knows that the Handler is controlling it and that he recieves a nick when he doesnt do something you expect him to do. This usually takes a few sessions to where the dog completely understands the ecollar is you telling them that the did something wrong.

Try letting your dog wear the collar for a little while, while he eats, while you play, while your doing fun things... so that he isnt shy of the collar.

Then move into using it for reinforcement in training. Command a sit, say he doesnt sit Immediately, hes a little stubborn, give him a low impulse nick. He will probably yelp, spin around, try to get away from whatever just bit him. Maybe fight to get the collar off. Hold him, command sit, if no response, a nick. Same reaction but now he is beginning to associate the command, his lack of response, and the nick together. In a few days the dog should know what the nick means.

Good luck.