Amateurs and novices in the making of documentaries can't resist illustrating every significant word in the commentary by cutting to a picture of it. The Lord Privy Seal is an antiquated title in Britain's heraldic tradition. The joke imagines a low-grade film director who illustrates it by cutting to a picture of a Lord, then a privy, and then a seal.
Crappy Documentary "I've hit rock bottom" (show a picture of a stone and a butt on the screen)
Audience, "Hey, that's another crappy Lord Privy Seal"