Ces expressions sont à l'impératif, donné comme un ordre. En version plus polie, on dirait "Pourrais-tu te taire" ou "Pourriez-vous vous taire"

@Li and Simone It is not offensive but it is directive. So it's perfectly acceptable to say it, but people might get irritated anyway (I guess it's exactly the same in english, it's not a matter of language). You can use : - un peu de silence, svp. This is an indirect way to ask to shut up, which is a smoother.