What does this sign mean?

It differs from a compulsory red-circle limit. Still obey the road limit and choose a safe speed, lower in rain or poor visibility.

黄色建议速度牌不是红圈强制限速牌，但仍须以安全速度行驶，并遵守该道路的强制限速；雨天或视线不良时可能需要更慢。