An immobiliser is an electronic security device which stops a vehicle from being started unless the correct digital key or token is present. In other words the car can only be started with it's key, and hotwiring it would be no use. On a mechanical level, immobilisers work by disabling at least two of the three main components which make your vehicle move, namely the ignition, fuel system and starter motor. Disabling two of these things will prevent a vehicle from starting. When you put your key in the ignition, a code is sent from a transponder in the key itself to the vehicle's electronic control unit(ECU), often via receiver in the teering column. The method will vary from one manufacturer to another, but they all work on the premise that the vehicle will only start if the key's code matches that of the ECU. The inner-workings of these devices understandably aren't widely publicised as this would give thieves the chance to get the upper hand. If the alarm/immobiliser has not been fitted properly or is faulty it will need to be replaced.