Final answer:
The yellow sign with an 'x' and two 'r's on each side indicates a railroad crossing, warning of potential train traffic ahead.
Step-by-step explanation:
The yellow sign with an x and two r's on each side is commonly understood to be a railroad crossing warning sign. This sign is crucial for drivers and pedestrians as it signals the presence of a railroad track ahead. Extra caution must be exercised when approaching this sign, as it indicates that trains may be crossing and there could be possible obstructions on the tracks. The sign's appearance is designed to be highly visible and universally recognizable to ensure safety around train tracks.