An azimuth mark is a station that is associated with a primary station. The important aspect of an azimuth mark is that its bearing from the main station is based on true North. This bearing from the main station to the aziumuth station can then be used by other surveyors to align their equipment based on true North without having to make complex measurements to determine true North.
Generally, an azimuth mark disk will say "AZIMUTH" on its rim and there will also be an arrow across the center of the disk that points toward the primary station. The disk's designation stamping might contain "AZ" as well.