“For Aaron’s sons you shall make tunics, sashes, and headdresses. You shall make them for their glorious adornment. You shall put them on your brother Aaron, and on his sons with him. You shall anoint them and ordain them and consecrate them, so that they may serve me as priests. You shall make for them undergarments to cover their naked flesh. They shall reach from the hips to the thighs. Aaron and his sons shall wear them, when they go into the tent of meeting, or when they come near the altar to minister in the holy place. Otherwise they will bring guilt upon themselves and die. This shall be a perpetual ordinance for him and for his descendants after him.”
You have the same clothing for Aaron and his sons. They are going to be anointed, ordained, and consecrated as priests. However, they should have undergarments to cover their naked flesh from their hips to the thighs. This is one of the first indications of undergarments since it was not clear what people wore. Perhaps the famous loin cloth was what we might call underwear today. Aaron and his sons were to wear these whenever they went into the meeting tent and the holy place. This is a perpetual ordinance for Aaron and all his descendents. Obviously the descendents of Moses are not that important.