Not that he may not sin, because that is impossible, but that he knows and understands his own sin. This is amazing, because it should produce a high priest or a priest that can relate to the people that he is offering sacrifices for. He should never look down on them, because he knows what the people are going through. He knows what it's like to be human. And so when they come to offer their sacrifice, the priest should take it, and he should look at them and say, "I know what that is like.
I can sympathize with you." And then he can go and offer it to God.