Forehead Protector (Headband) | Naruto

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A forehead protector (額当て, Hitai-ate) is a headband composed of an iron metal plate and a band of cloth. Forehead protectors are worn by most shinobi and are engraved with the symbol of their hidden village. ...
Forehead protectors are usually worn as a sign of pride and fealty for one's village. Ninja consider it an honourable, important part of their tradition, and make a point of wearing it at all times, especially when sparring with a comrade from the same village. Although not wearing a forehead protector is frequently viewed as disrespectful to one's village and peers, a ninja who does not wear one may still be loyal to their village. ...
Akatsuki members who retain their forehead protectors carve a long, horizontal scratch across the symbol, to show they have rescinded their allegiances and rejected their villages.

Naruto
Twelve years before the start of the series, the ninja village of Konohagakure was attacked by the Nine-Tailed Demon Fox. The village was only saved from complete destruction by the actions of Konoha's leader, the Fourth Hokage, who sacrificed his life to seal the Nine-Tails into a newborn: Naruto Uzumaki. Orphaned by the attack, Naruto grew up shunned by most of the villagers, who hated and feared him because of the Nine-Tails trapped within him. Unaware of the monster he contained, Naruto longed for acknowledgment, and so vowed to one day become the greatest Hokage the village had ever seen.

(from Narutopedia)