Forehead protector (Earth-2356)

A forehead protector is a headband composed of a metal plate and a band of cloth. Forehead protectors were worn by Jedi and are engraved with the symbol of their Jedi Order sect. Forehead protectors have been worn since at least the time of Old Republic, though there was no engraving then.

Meaning
Forehead protectors are usually worn as a sign of pride and fealty for the Jedi Order or one's own Jedi sect. Jedi considered it an honourable, important part of their tradition, and made a point of wearing it at all times, especially when dueling with a comrade from the same sect. Although not wearing a forehead protector was frequently viewed as disrespectful to one's village and peers, a Jedi who did not wear one might still have been loyal to the Jedi Order. Similarly, to have one's protector removed was to denote one's unworthiness to the name "Jedi". When a Jedi was to be stripped of their position from the Jedi organizational system due to a transgression of the Jedi Code, they were to relinquish their protectors as well.

Some rogue Jedi, such as Zabuza Momochi, continued wearing their forehead protector even after they've left the Jedi Order. Akatsuki members retained their forehead protectors, but carved a long, horizontal scratch across the symbol of their sect, to show they have rescinded their allegiances and rejected the Order and the Jedi Code. Inhabitants of Ame aligned with Pain wore forehead protectors out of loyalty and bore the same Akatsuki scratches across them; it symbolized their victory during the civil war against Hanzo.

Source

 * https://naruto.fandom.com/wiki/Forehead_Protector