How do ESD monitors work?
An ESD monitor or ESD tester like this:



is like a crude resistance meter.

ESD (Electro Static Discharge) events are caused by charge that is trapped. This charge will try to find its way to ground as soon as it can. This might be through a sensitive device which can be damaged.

To prevent that damage we have to make sure that no charge buildup can take place. That is done by connecting everything to ground including humans. The connection doesn't need to be very low ohmic (have a low resistance), even if there's a 1 M ohm resistor in series between the human and ground, that will still work.

Actually having some resistance in place between the human and ground is preferred as that prevents a large current from flowing when the human accidentally touches a mains live connection. Would you like to touch 240 V AC while your other hand is directly grounded? Neither would I but if the ground connection has 1 Mohm in series it becomes a lot safer.

That's that the ESD test station tests. It measures the resistance between your body (you need to touch a metal pad with your finger) and ground. If you're properly grounded with a safe 1 Mohm ESD wristband the test station will indicate everything is OK. If no connection at all is measured then you will fail the test.

So: an ESD monitor/tester measures the resistance between your body and ground.