DVDs are ecoded with region codes to indicate what area of the world they are made for. These codes may be 1,2,3, etc depending on which region they are made for.
DVD player read these codes and if the disc region code does not match the region the device is made for, the player will not play the disc.
Region 1 is for USA so most DVDp players made for use in USA require region 1 discs.
Some DVD players called multi region DVD players can be set to play different region DVDs but they are typically more expr=ensive then one region players and the code must be changed each time a different region disc is played.

Most commercial DVDs have a region code that ties the DVD to specific regions of the world, as a measure of copy protection. If a DVD's region code doesn't match the current region code of your DVD player, you'll receive a warning that you need to change regions in order to play the DVD. You can get a DVD copy without region code using Leawo DVD Copy. This tool can deal with different kinds of protected DVD discs.