A yellow diamond sign with the symbol of a truck on a downward slope means:

The truck-on-downgrade warning sign indicates a steep grade ahead. Trucks especially should select a low gear before starting the descent to use engine braking rather than relying on service brakes, which can overheat on long downgrades. Passenger cars should also slow down and avoid riding the brakes. The sign often appears with a 'percent grade' indicator (e.g., '6% grade') and may be accompanied by signs warning of runaway truck escape ramps further down. Steep grades are a leading factor in commercial vehicle brake failures and runaway crashes, and the warning is for drivers to plan ahead, not respond mid-descent.