a sash: a long band of cloth wound around the waist over a tunic, worn as part of Israel's priestly dress and as a mark of the office its wearer holds.
Almost every time this word appears, it is in the instructions for making Israel's priestly clothing or in the ceremony where that clothing is put on. It names a long band of cloth wound around the waist over the tunic, holding the loose garment close to the body so a priest could work at the altar. Exodus 39:29 describes the one made for Aaron: fine twisted linen with blue, purple and scarlet yarn, the work of an embroiderer. Aaron's sons were given plainer ones of their own.
Wearing it was part of the job rather than decoration. In the ordination ceremony of Leviticus 8, Moses dresses Aaron and ties the sash on him, then does the same for his sons, and the whole outfit is called holy. On the Day of Atonement the high priest set the colored garments aside and wore plain linen instead, sash included, before going in behind the curtain (Leviticus 16:4). One thing to keep straight when reading those lists: this sash is a different item from the woven band of the ephod (חֵשֶׁב), which is named right beside it in Leviticus 8:7.
The single appearance away from the sanctuary shows what the sash signified. In Isaiah 22:21 God removes Shebna from the post of royal steward and hands his robe, his sash and his authority to Eliakim. The sash there does exactly what it does for the priest: to wear it is to hold the position.