Regional Code is an encoding that restricts which part of the world a DVD can be played. Discs without such code are labeled all region or region 0.
Commercial DVD Players sold in a certain area (i.e. North America, Europe, Asia) are subject to only play DVDs with the regional code specific to that area.
"The purpose of this is to allow motion picture studios to control aspects of a release, including content, release date, and, especially, price, according to the region." (Wikipedia)
0. -- Meaning "worldwide". Region 0 is not an official setting; DVDs zoned Region 0 either have no "flags" set or 1-6 "flags" set (cannot be played/can be played in regions 1-6)
1. -- North America; US Territories; Bermuda
2. -- Europe; Western Asia; Kingdom of the Netherlands; Egypt; Japan; Lesotho; South Africa; Swaziland; British overseas territories, French overseas territories; Greenland
3. -- East ans Southeast Asia
4.-- Oceania; Central and South America; Caribbean; Mexico
5.-- Africa; Central and South Asia; Belarus; Mongolia; North Korea; Russia; Ukraine
6.-- Mainland China
7.-- Reserved for future use (found in use on protected screener copies of MPAA-related DVDs and media copies of pre-releases in Asia)
8.-- International Venues (i.e. aircraft, cruise ships, etc...)
ALL-- ALL dics have all 8 "flags" set; Meaning they can be played in any region
(Wikipedia: "DVD Regional Code")