The Start-Stop function is a system many car makers now fit vehicles with. This system linked to the car controls (accelerator, clutch,…) stops the engine when the car is stationary and the gear (manual) on neutral.
A simple rule for using Start-Stop
If on your regular journeys your stopping time (so your “Stop”) are longer than 10 seconds then you are in a configuration where your Start-Stop allows you to save on fuel. The longer you stop, the more you save.
However, if you happen to stop for less than 10 seconds, it is recommended to deactivate your Start-Stop.
Find the whole post about using this function efficiently here.