Because of geo-political, and religious reasons some movies are not allowed to be shown to the indigenous citizens. So to help out with that DVD drive makers along with DVD movie houses, divided the world in to 5 regions. (I called them zones tonight because it slipped my mind as to what the official term is. BK got it though.)
What happens is the DVD movies are encoded with a region code that passes that on to the optical drive. If the regions match, the movie plays. If they don't match the movie doesn't play.
For some reason the optical drive can only have the region code changed 5 times. Most likely a way to defeat people getting out of region movies and just flipping the region code to watch them. I'm sure by now some one has figured out how to cheat that. But how is not for discussion here.