Region codes in this instance mean flags implemented in DVD-Video discs that determine the geographic area where the DVD-Video disc is being sold and where it can be watched. These codes ensure that one country doesn't get a DVD movie before the same movie is out in that country's theatres. In their corporate omniscience, movie studios have carved the planet into regions with each region having a specific code.
All DVD players and discs have region codes. A DVD player and disc must be of the same region or the disc will not play. If you want to watch movies from other countries, you need a multiregion DVD player. This will allow you to play any disc from any region.
The region controls are also implemented in PC's DVD-ROM drivers, normally in three levels. First of all, if the DVD-ROM driver is manufactured after 1st of January, 2000, the driver itself has physical locks implemented in it to permit playback of only specific region code. Secondly, all newer operating systems have region control measurements built-in. And finally, the DVD player software, such as WinDVD or PowerDVD, have region control measurements built-in.