This is from a saying or idiom we use in English. "Children should be seen and not heard." It's a very "old school" or antiquated as well as patriarchal in nature (in my opinion). It is talking about discipline inchildren's behaviour. In your instance I suppose it could be used in more of a social sense to *create mystery*, as it may be "cooler" to be seen rather than heard. The minute one talks, the mystery and elevated viewpoint that has been constructed may simply disappear. In summary: 1) a good child is seen (in the view of their parents or guardians) and not heard (i.e. being quiet and not disruptive to the adults space). 2) in a social situation, it is better to be seen on the scene (think of a model, models are all about the visual and outward appearance in general) and not heard (because perhaps many models are not smart, so if they open their mouths: they become less attractive and lost their charm or mystery). I hope this explains :)

It is a proverb used often during Victorian Britain that usually goes “Children should be seen and not heard” and it means that children should not speak, sit quietly and behave well, especially when around adults. Hopefully it makes sense :)