The Breastplate of Judgement described in the Old Testament is one of the most fascinating artifacts found in all of scripture. I find it as compelling as The Ark of the Covenant, Liahona, Staff of Aaron, The Stone Tablets with the 10 commandments, the Urim & Thummim and other Seer Stones.
The Breastplate of Judgement was part of the clothing that would be worn by the temple high priest from the time of Moses and Aaron all the way down until it is apparently lost or no longer discusses after the Babylonian invasion in 587 BC.
Aaron is the first high priest with his Sons being priests in the temple as well.
The High Priest was the one who would wear the special clothing. Only once a year on the day of atonement, also known as Yom Kippur, the High Priest could enter the Holy of Holies and make an offering for the sins of all the children of Israel.