יעשית ציץ זהר טהור. ופיתחת עליו פתוחי חתם קדש להי.
-And you shall make a frontlet (headband) and engrave upon it "Holy to G-d." -Exodus 28:36
What about this headband is so special, and why does the Kohen have to engrave that specific phrase on it?
The Talmud (Arachin 16a) points out the juxtaposition between the priestly garments and the animal sacrifices. This juxtaposition is supposed to represent the atonement brought by the sacrifices and the priest. The Talmud elaborates upon each article of clothing further, and says that the frontlet is supposed to atone for brazenness.
Since the Kohen was acting as a liaison between G-d and the people of Israel, particularly when it came to atonement, I think the sin of brazenness applies to all the people of Israel, even the Kohen Gadol (High Priest). That might sound sacrilegious when first hearing it. After all, the High Priest has a lofty duty. Surely, he wouldn't screw up. But let's think for a second. It's not as if the Kohanim were unaware of their vocation. The Kohanim knew they were the priestly class that served G-d. Then why require a headband that says "Holy to G-d." Doesn't that seem unnecessary given what they already know? At best, it seems redundant or superfluous.
Yet it is precisely because of the nature of the Kohen's profession that the headband was necessary. One is supposed to wear tzitzit (fringes) to remind them not to sin (Numbers 15:38). Seeing the tzitzit is supposed to influence our behavior (Talmud, Menachot 44a). Even a King is supposed to carry two Torah scrolls to remind him of the power he wields. Tying the tzitzit and the King's commandment to carry two Torah scrolls together, what we see if that the Kohanim are held to similarly high standards, at least in part because power corrupts and absolute power corrupts absolutely.
According to R. Raphael S. Hirsch, the gold of the priestly garments represents the purity of heart. Purity means 100 percent--no abnormalities or contamination. To do his job, he needs to be of pure mind and pure heart, which is ironic considering it needs its own brand of brazenness and impudence. The priest, like the rest of us, is all too human. Even with the loftiness, there still needs to be that external reminder of his mission: to be "holy to G-d." Given the power that the priest has and the standards for the priestly vocation, the extra reminder provided by the golden headband is all the more relevant.
What does that mean for Jews in the twenty-first century? As former British Chief Rabbi Joseph Hertz points out, the golden headband is supposed to crystallize the aim and purpose of the priest's service. Much like the priests in past times, we need to provide ourselves many external reminders: tzitzit, tefillin, mezuzah, the list goes on. The Jewish people are meant to be a priestly nation (Exodus 19:6). This goes back to the idea of the Jews being a chosen people and what that really means. Being chosen doesn't mean being superior, but rather having additional responsibilities. Jews are supposed to do mitzvahs, study Torah, pray, do acts of loving-kindness, all of which bring about many directives and expectations. The goal of the Jewish people is to be a light unto nations (אור לגויים). The golden headband reminds us that even if our purpose seems clear as day to us, we still need all the constant and blatant reminders of our mission on G-d's green earth than we can get.