“The herald of coming good” means a sign, messenger, or early indication that something positive is about to happen.

A herald is literally someone who announces news in advance. So this phrase frames good fortune as something already on its way—you’re just seeing the first ripple before the wave arrives.

Conceptually, it sits somewhere between:
omen (but positive, not superstitious)
harbinger (without the usual doom)
early signal (with a lyrical sheen)