I'll give it a whirl. Different countries have different regions. Here's a list that breaks it down for you:
0 No Region Coding
1 United States of America, Canada
2 Europe, including France, Greece, Turkey, Egypt, Arabia, Japan and South Africa
3 Korea, Thailand, Vietnam, Borneo and Indonesia
4 Australia and New Zealand, Mexico, the Caribbean, and South America
5 India, Africa, Russia and former USSR countries
6 Peoples Republic of China
7 Unused
8 Airlines/Cruise Ships
9 Expansion (often used as region free)
Why all the regions? Copyrights! Dvds are encoded with "flags" that enable the player to recognize what country the DVD is from. The idea is that you can't buy a copy of Lord of the Rings from England (or most likely China) and pop it in your DVD player in the US of A and watch it. Why? DVDs cost more in some countries, movies are released at different times in different countries, piracy, etc... A region free player will play DVDs from all other regions. When you buy your DVD player from the store the odds are very good that it has been locked to only accept DVD's from your region. Sometimes this is a firmware lock, meaning you need to modify the internal hardware of the device (can be as simple as flipping a switch), and sometimes it is a software lock, meaning there is a code you can input into the player to make it region free. Having a region free player is only important if you plan to watch movies not from your native region. Another thing you need to be wary of is PAL VS NTSC. But they are for another thread.
Hope it helps, good luck.