风向标 literally means 'wind vane' — a physical instrument that shows wind direction. But in modern Chinese, it's far more common as a metaphor for a trendsetter or indicator that signals the direction of change in fashion, politics, or public opinion. You'll see it in news and analysis: 市场的风向标 ('market bellwether'), 时尚的风向标 ('fashion barometer'). The literal sense is rare in everyday speech; the figurative sense dominates.