If you search the forums you'll find a few different descriptions. Some describe persistent flickering, making the screen look like it is pulsing brighter and darker. The best analogy is an old CRT screen out of sync. The cause seems to be the 2G/EDGE radio interfering with the screen. If you live in an area where you will not have HSPA/H+ or LTE, this might be a concern. This happened to me when I was in parts of Italy and it was pretty annoying, but hasn't happened since I got back to the US (I'm using my 930 on AT&T).
Others report the screen flashing black for a 10th of a second. I don't think I've ever experienced this. Sometimes, when viewing content on a bright page, the screen seems to 'flash' when it redraws - like when an ad gets changed on a mostly white web page, but the whole screen redraws not just the ad.