Blu-ray discs are sometimes encoded with what's called a region code.
The aim of this is to try and restrict the countries of the world which can play a specific disc.
If you're familiar with DVD region codes then this should be nothing new, but the geographical areas are slightly different. Blu-ray players purchased in a specific region can only play the discs encoded for that region.
This helps Hollywood studios to control and restrict a particular release of content. There are howevere some discs that contain no region encoding, which means they can be played on all devices, no matter where they are located or were purchased.
By early 2008, about 2 thirds of all Blu-ray discs were region-free.
Some Blu-ray players have been modified to allow for playback of discs from Regions A and B and DVD discs from Regions 1 and 2.