They're soft subtitles. The exact position can differ from player to player.

Subtitles are stored as four-colour full screen images, so they should always appear in the same place on all players, which in this case will be bad if the subs are outside the 16:9 frame and people want to zoom to fill their wide screens (some TVs have a "subtitle" zoom for exactly this situation).

The best use of this facility I've seen was on the Ghostbusters DVD. There's a subtitle track which takes the form of an MST3K-style silhouette of the people doing the commentary