Herald is the title given to a chosen person by one of the Great Three Mystical Beasts, the supreme creatures that can't be summoned. Due to the fact that nobody can summon one the Three Great Beasts due to their inmense power, the Great Beasts can choose people instead and charge them with a little bit of their power. However, this tiny portion of power is more than enough to make a person nearly invincible. The mission of the heralds isn't only to transmit a message as their title may refer. These chosen people are given highly important missions that most of the time involve the safety of all mankind. Officially the Heralds are know as a myth, like ghosts or fairytale creatures, due to the fact until now, nobody had ever heard or seen of any Herald. People sometimes even confuse them with the Jinchuriki, and finally Heralds were discard as an old crazy tale. The legends says there are three requirements for being choose as Herald: 1. You must have pure heart, an unstoppable courage and a wise and clear mind. 2. You must be able to quit to everything and to everyone in this world. 3. And finally you must die.
Isn't clear if these requirements are the offical ones or just part of the legend. Neither for the Heralds themselves is clear why they were chosen in the first places, they think that the Great Beast look more than just the main requirements, they see something inside them that anyone else sees, neither the Heralds themselves. The only Heralds in this moment are: Hikari: The Yang Herald of the Dragon Emperor / Kagemaru: The Ying Herald of the Dragon Emperor / Shion: The Herald of the Black Fire Phoenix / Kya: The Herald of the Sacred White Tiger
As shown with Kagemaru, Shion and Kya, you don't have to be talented, powerful or even a shinobi to become an Herald, making the process of selection more mysterious. These chosen people keep their identities as Heralds secretly and even in cases like Shion and Kya they take new identities, but in cases like these, it mostly depends on the circunstances of their life and deaths.