It’s a blood vessel bypass to the whole brain, the BBB is between blood vessels inside the brain and brain tissue.

The implication for drugs, is in drug research: normally a drug will get spread all over the body, particularly passing through the liver, which metabolizes all it can, followed by the kidneys, which piss out all they can… so it isn’t easy to estimate how much of a given drug actually gets to the brain.

With a bypass, they can inject drugs directly into the brain, and see how they work without “interference” from the rest of the body.