Those yellow reflector lights are *side marker or clearance lights*. They’re used on vehicles (especially trucks or trailers) to:
1. *Indicate the vehicle's width* and improve visibility from the side.
2. *Emit amber light* when the headlights are on, helping other drivers see the vehicle's outline, especially in low‐light conditions.
3. *Enhance safety* by marking the vehicle's extremities to prevent collisions.