I first learned this term from a post by Richard Dawkins a few months ago in the aftermath of this incident. Here's what he said:
Wikipedia also has some information on this term.
A favourite joke among the film-making community is the 'Lord Privy Seal'. 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.
With the advent of cheap and easy video editing software, anybody can sit down and create a video to a song with very little effort. Some people do outstanding jobs - see, for example, Lauren's excellent video for her song "45" - but others do very literal slideshows of every term used in the video. Take a look at this video that someone put together for the Smashing Pumpkins' cover of Fleetwood Mac's (well, Stevie Nicks') "Landslide" - a cover that I feel is superior to the original and to every other cover that I have heard. But the video...meh.* Is it really necessary to show a picture of a landslide every time the word "landslide" is used?
I'm not going to complain too much. Without these Lord Privy Seal videos, a lot of this music wouldn't be on YouTube at all. And one such video pointed me in the direction of the video I grabbed for future use. But still...what makes people so literal-minded?
*This is not an official Smashing Pumpkins video. As far as I know, the Smashing Pumpkins never made a video to go with this song.