Matthew Henry Commentary Exodus 30:17-21

"And Jehovah spake unto Moses, saying, Thou shalt also make a laver of brass, and the base thereof of brass, whereat to wash. And thou shalt put it between the tent of meeting and the altar, and thou shalt put water therein. And Aaron and his sons shall wash their hands and their feet thereat: when they go into the tent of meeting, they shall wash with water, that they die not; or when they come near to the altar to minister, to burn an offering made by fire unto Jehovah. So they shall wash their hands and their feet, that they die not: and it shall be a statute for ever to them, even to him and to his seed throughout their generations." — Exodus 30:17-21 (ASV)

A large brass basin, holding water, was to be placed near the door of the tabernacle. Aaron and his sons must wash their hands and feet at this laver every time they went in to minister. This was to teach them purity in all their services and to dread the pollution of sin. They must not only wash and be made clean when first made priests, but must also wash and be kept clean whenever they went to minister.

This teaches us to approach God daily, to renew our repentance for sin daily, and to look daily to the blood of Christ for remission; for in many things we daily offend.