A start-stop system stops the engine when the car comes to a halt, for example, in front of a traffic light or during a traffic jam. The engine automatically restarts when pressing the clutch or the accelerator. Obviously, this saves fuel and reduces emissions.
Claim: This system saves money. Here is an example from the Volkswagen homepage:
We think that sitting in traffic is bad enough without knowing that you have to pay for it too. That's why we created Start/Stop Technology, a really simple way of helping you to save money whenever you come to a stop.
Reason to doubt: Cars consume little fuel when halting. Starting the engine more frequently might wear out starter motors or batteries faster, increasing service cost.