It’s a device that fools the engine’s control unit into thinking that AdBlue* is being delivered to the exhaust system, when it is not.
This causes the engine to emit illegal levels of NOx in the exhaust gas.
*AdBlue is an aqueous solution of urea. When injected into the exhaust the heat turns it into ammonia. The ammonia reacts with a catalyst in the exhaust and the NOx in the exhaust gas. The ammonia and NOx are converted into water vapour, nitrogen and CO2.