In writing, a herald is a person, thing, or event that calls the hero to adventure. It is the personification (or, as the case may be, objectification) of Joseph Campbell's Call to Adventure.
In the archetypical
plot, the hero is leading a fairly normal (or difficult) life, with no particular expectations of change. Then the herald comes, and explains why an adventure is necessary. Examples include
Bilbo Baggins and
Gandalf,
Montag and
Clarisse McClellan,
Harry Potter the
Hogwarts' owls,
Luke Skywalker and the two droids, and
Arthur Dent and
Ford Prefect.
However, in many cases the herald is not a person, but an event. In The Martian the herald is a sandstorm, in Sense and Sensibility the herald is the death of Henry Dashwood, and in Attack on Titan it is an attack by titans.
And at times, the herald can simply be an item (and this item is sometimes also a MacGuffin). In Half Magic it is a magic coin, and in Through the Looking-Glass it is simply a looking glass.
The herald is a highly flexible role, and as with all of the archetypes authors play around with them -- that's part of the fun of being an author. In an ODTAA plot there may be many heralds, and in complex plots with multiple heroes, they may share a herald, but they may not. Campbell notes that a common initial response to the Call to Adventure is The Refusal of the Call -- that is, the herald is often ignored or disregarded at first, until Hagrid knocks down your front door, Vogons blow up your planet, or stormtroopers murder your family.