A debug monitor is an atomic adapter.
It does not change the data flow: any input is passed through to the output unmodified.
Its function is to monitor the data flow and output some logging info to a log output somewhere. This could be a console log or a log file or some other form of log output.
Debug monitors can be switched on or off. During normal operations they'll normally be switched off, but when necessary, they can be switched on to help get an insight into the data flow, and help diagnose issues by giving an 'inside look' in the flow of granules passing through the adapter network.