A person or thing that announces or signals the approach of another. The blooming flowers are a harbinger of spring.
A forerunner of something. The economic downturn was seen as a harbinger of the financial crisis.
Historically, a person sent ahead of a travelling party, especially of royalty or an army, to arrange lodgings and other preparations. In medieval times, the harbinger would ride ahead to secure quarters for the king’s retinue.