Wikipedia
A schulklopfer is the person who calls a Jewish community to prayer in the local synagogue. The Yiddish term "schulklopfer" literally means "synagogue knocker". In modern times, the custom has more or less died out, but it was historically common.
The schulklopfer was usually a beadle, who would perform the task by wandering around the community, knocking on each household's door. In Neustadt, he would knock four times, in the pattern KNOCK - pause - KNOCK KNOCK - pause - KNOCK; Israel Isserlein (a famous rabbi from Neustadt) argued that this pattern was a reference to the Biblical phrase I shall come to thee and bless thee (in gematria, the letters of the first word of this phrase have the values 1, 2, and 1, respectively). In the Rhine, the custom was to strike merely thrice, in the pattern KNOCK - pause - KNOCK KNOCK.
In mediaeval Eastern Europe, the schulklopfer also had the role of individually inviting people to marriage ceremonies ( nissuin ); the invitations were made to the entire community by the schulklopfer on the morning of the marriage ceremony itself (such ceremonies were usually an evening affair)
The name stems from the Holy Roman Empire (Germany) in the middle ages. Christians in nearby communities sometimes referred to schulklopfers as campanatores (a Latin term meaning bell-strikers) or as Glöckner ( German for bell-striker); these terms were the Christian equivalents.