BD+
BD+ is a component of the Blu-ray Disc Digital Rights Management system. It was developed by Cryptography Research Inc. and is based on their Self-Protecting Digital Content concept. Its intent was to prevent unauthorized copies of Blu-ray discs and the playback of Blu-ray media using unauthorized devices.
BD+ mainly works by adding errors to the video stream, not enough to make it unwatchable but enough to make it unpleasant to watch due to near constant artifacting. These are fixed in official players by using "fixup tables", which are downloaded from the internet and provide a mapping to convert the broken video stream into the correct video stream.
You can determine whether a disc contains BD+ protection by checking whether the /BDSVM directory exists.