BD+ is a part of the Blu-ray Disc Digital Rights Management (DRM) system, intended to prevent unauthorized copies of Blu-ray discs and the playback of Blu-ray media using unauthorized devices.
It is basically a small emulated processor with its own memory area and hooks into the machine that so that the DISC can load its own code into 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 then only allowing decrypting with a program from the disc itself which is loaded into the BD+ engine. This has been done on various discs already.
The 650BD/651BD/751BD/752BD are obviously fully compliant units and fully supports the BD+ spec.