The mitznefet is one of the eight items of the Bigdei Kehuna (priestly clothing) that the Kohen Gadol (high priest) wears during his service. It is a hat made of linen and is sixteen amot (handbreadths) long. The miztnefet is worn in place of the hat of the Kohen Hedyot (regular Kohen) called a migbaat. The hat was wrapped around the head of the Kohen Gadol and then sewn together to maintain its form.
Fun Fact: According to Chazal (the Rabbis), the hair of the Kohen Gadol was visible between the miztnefet and the tzitz (golden crown). There the Kohen would put his tefillin (phylacteries) every morning.