Watchers are Fault Detection and Diagnosis (FDD) Rules. FDD is the process of identifying (detecting) deviations from normal or expected operation (faults) and resolving (diagnosing) the type of problem or its location.
Fault detection and diagnostics tools such as the Watcher tool basically monitor the data points on your site from energy metering to the control system in real-time (temperatures, flows, pressures, actuator control signals, etc.) and then apply a set of rules.
These rules are applied in the form of logic groups and can be as simple or as complex as required.