What is it?
Although lurking is normally a negative term, on the internet it means to read or follow an online discussion without taking part in it (such as by adding comments).
Why is it useful?
Lurking can be useful to take time to understand a discussion and the site's netiquette (acceptable behaviour and communication style) before commenting yourself. In general, there are far more people observing than taking part.
What do you need to keep in mind about your child and lurking?
Lurking on the internet does not have a sinister meaning like stalking does, however you might wish to explain the term to your child, and also the benefits of taking part in a discussion, and at other times sitting back to observe it.