to treat someone as great, and show it: to honor a person by what you visibly do in their presence, or to be arrayed in splendor yourself.
Behind every use of this verb stands one picture: greatness that can be seen. It is not a quiet inward esteem but the visible act that puts a person's weight on display, and in Hebrew it usually attaches to someone's face, meaning the way you carry yourself in front of them. You stand up when an old man walks in. You dress so that anyone can tell who you are. You either give a person their due in public or you withhold it.
That is why the same act can be commanded and forbidden within one chapter of Leviticus. Verse 15 warns a judge not to honor the face of the great man, because a powerful person's standing must never bend a verdict. Verse 32 commands the very same act toward the elderly. The word itself carries no verdict: who is receiving the honor, and why, is what makes the act right or wrong.
Two uses stretch the picture in opposite directions. In Isaiah the figure who comes from Edom is himself splendid in his clothing, so here the honor is worn rather than given. In Proverbs the warning is against doing it to yourself, pressing forward into the king's presence as though you were the important person in the room.
The noun built on this same root (הָדָר) is the splendor itself, the dignity a great person displays. This verb is what people do about it.