In Tanach, the forehead is used as a metaphor for the trait of brazenness. When Yirmiyahu rebukes the Jewish people for their idolatry, he tells them “you had the brazenness (מצח) of an immoral woman”. The word מצח literally means a forehead.
The commentaries explain that an uncovered forehead is a sign of brazenness and Chutzpah. Whereas a covered forehead is a sign of humility and modesty
Our sages teach that the Tzitz, which was worn was worn on the מצח – forehead of Aharon, atoned for the sin of brazenness. The sin of brazenness is that despite knowing better, one acts audaciously with disregard, doing as they wish.
The Tzitz covering the forehead is a sign of humility. The word Tzitz also means to gaze, representing a constant of awareness of the presence of Hashem. This awareness brings us to humility and surrender.
On a deeper level, brazenness, like all middos, can be positively used in the service of Hashem. This is the directive in Pirkei Avos to be עז -brazen - like a leopard… to do the will of your Father in Heaven. We do this by being undaunted and unashamed in the face of mockery or challenge to our faith and keeping of Torah and Mitzvos.
This is the hidden meaning in Moshe’s describing Bnei Yisroel as a stiff-necked people. Deep inside, every soul possesses an absolute, irrational and unyielding dedication to Hashem that we need to actualise.
The Tzitz is a reminder that we need to harness this holy brazenness, that our ‘forehead’ should be Kodesh LaHashem.
~ Based on Or Hatorah Parshas Tetzaveh p1756
 Yirmiyahu 3:3
 Metzudos Dovid
 Zevachim 88b, Shir Hashirim Rabbah 4:4