When one thinks of the stereotypical ninja, loyalty isn't usually one of their typical traits. The ninja of Naruto's Hidden Leaf Village are no stereotypes, though. They work in teams of three or more, rather than alone, and prize loyalty to one another and the village above all else. This is their Will of Fire.
The Will of Fire is an idea shared among Konoha's villagers that love and loyalty are ideals that should be upheld above all else. Anything less is shunned and changes how the villagers view the offending shinobi. This ideal turned is hypocritical with how the villagers initially treated Naruto, who became an object of their hate and distrust as the Nine-Tails jinchuriki. However, it was Naruto who would go on to embody this idea so much that he would eventually become Hokage, a beacon of love and loyalty for even those who once hated him.
The Will of Fire originates from Asura Otsutsuki, son of Hagoromo Otsutsuki, The Sage of Six Paths and precursor to the shinobi. Asura and his brother Indra were both trained in the art of Ninshu, the early form of ninjitsu, although Indra was more talented than his brother. However, Asura pushed through his limitations and it was his ability to connect to others through love and loyalty that earned him the right to his father's legacy. Indra was angered by this and declared war on his brother. Their feud would go on as long as they reincarnated. Indra would go on to reincarnate into Madara Uchiha, his descendant. The Uchiha Clan would inherit his Curse of Hatred and Indra's ability to use the Mangekyo Sharingan. Asura's descendants, the Senju Clan, would inherit his Will of Fire. He would reincarnate into Hashirama Senju. It's worth noting that in Hindu and Buddhist beliefs, the gods fight the Asuras, with the gods being led by Indra.
Madara and Hashirama eventually found the Hidden Leaf Village, Konohagakure, but eventually continue the feud between the two wills. The next ones to inherit the chakra and will of Indra and Asura would be Sasuke Uchiha and Naruto Uzumaki, respectively. They would be the ones to break the cycle of fighting and find peace between the two, as Sasuke accepted that Naruto's ideals were superior to his own after being defeated in battle.
This was the message of Naruto and the Hidden Leaf Village. Anyone who went against these ideals, like Orochimaru, were despised by the village and generally antagonistic. A few members of the Uchiha Clan were able to overcome the Curse of Hatred, with Itachi being one of them, surprisingly. Sarada has also inherited the Will of Fire rather than the Curse of Hatred, meaning that, hopefully, the Curse ended with Sasuke.
Now that the war between ideals has ended, Asura and Indra's spirits should be able to rest in peace after Naruto and Sasuke pass, being their latest reincarnations. Naruto was able to show Sasuke that love and trust are how one truly becomes powerful, and that hatred and distrust only leads to pain and misery. The Will of Fire is what gives the villagers hope and the ability to face any hardship. That's how they've become strong.