Its idiomatic here but a deer is an animal. If you are driving at night, the headlights on your car are usually on so you can see where you are going. So you are driving at night with yourheadlights on and suddenly there is a deer on the road in front of you. Your lights are shining directly on it. It will be so frightened that it will be trapped and unable to move. That is the meaning here -very frightened and trapped.