For Moslems, Friday is a weekly holy day. In that holy day, Moslems go to masjids and mosques to do a special worship called Jumatan.
There are two sessions in Jumatan: khotbah and shalat. Moslems are required to fully listen the khotbah for the whole prayer to be accepted by Allah. For that reason, they have to remain silent and listen throughout the khotbah.