BD+ is a part of the Blu-ray Disc Digital Rights Management (DRM) system, intended to prevent unauthorised copies of Blu-ray discs and the playback of Blu-ray media using unauthorised devices.
It is basically a small emulated processor with its own memory area which hooks into the machine so that the Blu-ray disc can load its own code independent of everything else.
The idea is a that a disc can load in programs to check the player for hacks, etc. The machine has no control over it, only the disc.
The use of BD+ can also include encrypting the disc, only allowing decryption by a program within the disc itself which is loaded into the BD+ engine. This has been done on various discs already.
The CXU is obviously fully compliant and fully supports the BD+ spec.