A more thorough explanation:

A synodsman, also known as a sidesman, is a church officer who used to report to the bishop about any wrongdoings by the clergy or congregation, including heretical acts. Over time, the duties of a synodsman were taken over by the churchwarden.

For example, if a member of the congregation was suspected of committing a crime, the synodsman would investigate and report it to the bishop. However, if the synodsman was not available, the churchwarden would take on this responsibility.

The term "synodsman" comes from the word "synod," which is a meeting of church officials to discuss important matters.