It simply means your child has to understand it. You would not let your child cross the street without teaching him or her to look both ways first. The same applies here. Make sure that your child understands that using the Internet is similar to communicating with someone on the phone. It is not just a machine.
Real people are the core of it.
They tell it what to do. Instruct your child not to talk with people in chat rooms unless you say it's okay. And if you do make sure you review the situation first. If your kids look for information through news groups tell them to read but not to reply, unless you say it is okay. And if you do, remember that anyone can read your reply and contact you through your email.