I'll give you an example.

Let's say a character in a movie is cooking an egg.

Normally it's part of a breakfast scene. Maybe the character is spacing out and having an emotional moment, maybe they're just cooking an egg. Either way, there's not much to the action in the scene.

But your pretentious director might use dramatic camera angles go for close ups of the eggs or something else, place dramatic music into the scene or otherwise amp up the presentation in a way that tells you the viewer there's something to pay attention to.

They might think you're catching on to the theme of 'death, rebirth in the destruction of a cradle of birth' or something, but really all you see is...a person cooking eggs to weird camera angles and dramatic music.

Basically it's when a director is doing too much, trying to make simple or obvious things seem important.

The crazy thing is, when done well it's not pretentious, it's thought provoking. The spinning top at the end of Inception isn't just a spinning top, and the camera closing in and the dramatic music is justified.