What does the marking mean?

Correct answer is it may be a slow-moving vehicle.

In the United States and Canada, vehicles that are unable to reach a maximum speed of 65 kilometres per hour are required by law to display a reflective emblem sign at rear of the vehicle. The sign appears as a cut triangle consisting of a triangle with red markings cut of in red as a border surrounding an orange uncut triangle in the center. These vehicles are termed slow moving vehicles (SMV).

A vehicle driven by a student driver is likely to have a student driver sticker on the vehicle indicating that a student driver is driving the vehicle. Vehicles hauling hazardous materials are required by regulations to display a diamond shaped, square on point warning signs.