Most of our electric ovens have an indicator lamp next to the temperature selection control. Once the oven has reached the temperature you have chosen,then the light will go out.The pre heating is completed and the food can be placed inside the oven to cook.
If your oven does not have an indicator light please consult your instruction book. A copy can be downloaded from this website.
The light may come on again as you add the food or open the door to check the progress of the cooking. This is perfectly normal.