A reflectorised triangle placed on the side of the road is a warning device indicating a breakdown, accident, or hazard ahead. It is meant to alert approaching drivers to slow down and proceed with caution, reducing the risk of a collision. Key points to know (generally applicable across many regions):

• Purpose: Visual warning of a stationary vehicle or hazard on or near the roadway.
• Placement: Typically placed behind the vehicle on two-way roads and in front of the vehicle on two-lane or single-lane approaches, at recommended distances (commonly tens to hundreds of meters depending on the country and visibility conditions). In low light or poor visibility, the triangle should be placed further away to maximize visibility.
• Visibility: The reflector material makes the triangle highly visible when headlights illuminate it, even from a distance.
• Compliance: In many jurisdictions, drivers are required to carry and deploy warning triangles when a vehicle breaks down or is involved in a crash. Always follow local rules for required number, placement distances, and whether a triangle must be used on certain types of roads or in specific conditions.
• Safety tips: Turn on hazard lights, wear high-visibility clothing if exiting the vehicle, place triangles where they are visible to oncoming traffic but do not place them in a way that obstructs traffic or creates additional hazards.

If you’d like, specify your country or region and I can give precise rules for that location (e.g., how many triangles, exact distances, and recommended placement guidelines).