mattsl, I see you have only received wise-crack answers. Flipping the stem refers to removing the stem from both the steerer and handlebar, rolling it over so the logo now reads upside down and reinstalling, still pointing ahead. (There are stem makers who use labels with both right-side-up and up-side-down logos so you can flip them and not mess up their artwork.) This is done to change the handlebar height with only a minor change in the horizontal location. Doing this only makes a difference if the angle between the stem and steerer is not 90 degrees and makes little difference with short stems.

In 50 years around bikes, I have seen what you posted maybe 3 or 4 times. Maybe 1 or 2 that weren't by ignorance/accident.