Charles Ellicott Commentary Exodus 28:38

"And it shall be upon Aaron`s forehead, and Aaron shall bear the iniquity of the holy things, which the children of Israel shall hallow in all their holy gifts; and it shall be always upon his forehead, that they may be accepted before Jehovah." — Exodus 28:38 (ASV)

That Aaron may bear the iniquity of the holy things. —The “holy things” are the offerings brought by the people. These would always have some “iniquity” attaching to them, some imperfection, owing to the imperfection of human nature and the mixed character of human motives. The high priest’s official holiness enabled him to present to God offerings thus imperfect without offending Him. It was accepted as purging the offerings from their impurity.

It shall be always upon his forehead —that is to say, during his ministrations.