Arachnoid granulations are small protrusions of the arachnoid mater (one of the three meninges that cover the brain and spinal cord) into the dural venous sinuses, which are channels that drain venous blood from the brain. They act as one-way valves that allow cerebrospinal fluid (CSF) to exit the subarachnoid space and enter the venous system, thus playing a crucial role in regulating CSF pressure.