It is a metaphor that means to be frozen in fear or indecision in a moment of crisis or some difficulty just like a deer at night crossing a road when a car suddenly appears with headlights on. It's one of the responses an animal in danger can have: fight or flight...or in this case to freeze when confronted with a blinding light. Of course, it's a fatal response in this situation. It happens to people too. Does this answer your question?