Azor Ahai is the warrior that forged the sword Lightbringer and drove the darkness away. During the Long Night, Azor was chosen as the hero to end the darkness. In order to defeat this darkness, he needed a powerful blade. He was able to forge such a blade after a few tries, though it took
him one hundred days and one hundred nights to make.
On the hundredth day, he plunged the blade through his wife Nissa Nissa’s heart. Her soul combined with the hot blade and created Lightbringer. The fiery weapon drove the darkness away, thus ending the Long Night.
It is believed that Azor Ahai will be reborn again, sent by Rhllor, the Lord of Light. In the prophecy, the new Azor Ahai will rise from smoke and salt, as the "prince that was promised." The Song of Ice and Fire series elaborates the prophecy concerning who would be the prince for the upcoming long winter and deliver the world from darkness.
There are a number of characters in the Game of Thrones series that fit the bill. Daenerys Targaryen and Jon Snow are rumored to be the prince in the prophecy as both are born from smoke and tears.