Three parts make up an automobile immobiliser: a control unit, a relay unit, and a key. An electronic immobiliser prevents a vehicle from starting unless the proper key or key fob is used. When you start your car, the key or fob sends an electronic code to the ECU, which is how car immobilisers work. The engine will, after that, be able to start.
However, the immobiliser would activate and stop the ECU from getting that signal to start the engine if the car is being stolen and the burglar tries to use a different key or fob. This is why fixing a broken car immobiliser is crucial as soon as you identify the problem.